Sepiolite is a natural hydrated magnesium silicate clay mineral with microfiber structure and high surface area. Fibers are constructed by tetrahedral and octahedral structured oxides within half cell formula of Sii2O30Mg8(OH,F)48H2O. Sepiolite has special futures like; high surface area, fiber structure, porosity, tunnel/channel composition, crystal morphology, surface activity and formation of stable high viscosity suspensions at low solid concentrations. This unique structure of sepiolite can be altered by heat and/or acid activations. It also exhibits severe sensitivity to any mechanical perturbations; any interference may result as structural changes which can enhance or deteriorate the characteristics of sepiolite.Cation exchange capacity, fiber dimensions (fiber length/fiber width) and the type and amount of accompanying impurities are the main factors identifying the reologic behavior of sepiolite pulps. If sepiolite fiber bundles are disintegrated in water with a mechanic impeller, these particles may form random structures in the form of networks and increase the apparent viscosity values. Better dispersion of these fibers leads to an enhancement of water sorption on the fiber surface cmd in turn in inner galleries, resulting in entrainment of water within the network and thus viscosity increase.In this study, the dimensions of sepiolite fibers after mechanical dispersion are investigated based on a technique described in the literature for monitoring the sizes of mohtmorillonite and smectite type clays by Atomic Force Microscope (AFM). Unlikely the previous studies where a sepiolite fiber sizes were measured with SEM technique; conducting AFM study to determine the difference in the sizes of sepiolite fibers is unique. The mechanical dispersion process is conducted as a function of time and stirring rate, and the results of rheological behavior is followed through viscosity.A Turkish sepiolite sample of 65+10% purity was obtained from Eskişehir region. Sepiolite suspensions were prepared at 3%solids by weight in pure water as described in the literature. As sepiolite suspensions are characterized by time dependent flow behavior, they exhibit thixotrophy. The viscosity measurements were taken at different spindle rates but particularly at 5 rpm spindle rate. The highest viscosity value was obtained with the suspensions prepared at the highest stirring rate of 21.000 rpm. The samples run at 1, 3, 5 min. cmd the corresponding viscosity values of 6400, 9600, 4800 cP respectively were collected for AFM investigations. Several techniques have been established to investigate platelet structured clays. For our special case, these procedures were overrun and modified to investigate the fiber structured sepiolite samples; this involves dropping of 1 ml dilute sepiolite suspension onto a clean mica surface. The drop is dried with nitrogen gas and the mica plate is stored in a Petri dish till AFM investigation to avoid any airborne impurities to settle on the surface.For AFM investigations, Nanoscope III model AFM set up was used at the tapping mode to produce 512*512 pixel graphics of sepiolite settled on the mica surface. The sample prepared for 1 minute stirring, represents bundles consisting of approximately 50-60 fibers. Ensuing sample of 3 minutes stirring was made with fiber groups of 5-10. Increase in apparent viscosity values upon increasing stirring time, indicates the presence of loosened bundles and thus better network. However, when the stirring period is increased to 5 minutes the viscosity value decreases and the AFM graphics, showed the failure in size uniformity with accompanying a weak networking.AFM graphics n-Surf 1.0 Beta visualizer program coupled with XYit program is used for numerical analysis of the fiber or bundle dimensions. Finally the collected data are averaged with Excel program. Investigating a total of 130 profiles, the average height*width of 1, 3 and 5 minute stirring is found as 514*50, 249*28 and 157*22 nm, respectively; the corresponding fiber or bundle lengths are 1643, 1126 and 1055 nm, respectively. As a result considering the average sizes, at the same percent solids, the number of fibers or groups in a suspension prepared at 3 minute stirring is 8 times more than that of 1 minute stirring, indicating a better network leading to higher viscosity. As opposed to this, further stirring at 5 minute stirring deteriorated the dimensions of fibers and the formation of network due to fracture of fibers.