Sivas basin is the one of Central Anatolian Basins occured after the closure of the northern branch of Neptethys in the uppermost of Cretaceous. Sivas basin sediments contain marine and continental sequences having from Cretaceous to Pliocene ages. Oligo-Miocene sequence in the central part of the Sivas basin was deposited in three different environments as open sea, lagoon and lake based on the geologic mapping and biostratigraphic data. In this study, the relationship between Oligo-Miocene paleo-environments and clay mineralogy has been investigated by means of optical microscopic and XRD methods. An association of clay (smectite ± illite or ± mixed-layered illite-smectite/l-S) + calcite ± quartz ± feldspar is present in the marls of the open marine environment. In the lagoon environment sediments with lateral transiton to each others are subdivided into two levels. Siltstone and sandstones representing lower levels contain clay + calcite + feldspar + quartz ± analcime. However, the upper level is characterized by clay (smectite ± illite ± I-S ± chlorite) + gypsum + calcite ± quartz ± opal-CT ± feldspar in the clay laminated gypsums and gypsum-bearing marls and calcite + clay (kaolinite + smectite ± I-S ± illite) ± quartz ± feldspar + gypsum in the marls rich in organic material. The assemblages of smectite + palygorskite ± illite ± chlorite ± I-S and C-S) + dolomite and/or calcite + feldspar + quartz are recognized in the lacustrine marls. Environments as open marine, lagoon and lacustrine are dominated by diagenetic mineral suites such as smectite and calcite; gypsum, calcite, smectite and kaolinite; and dolomite, palygorskite and smectite, respectively. Units with similar appearences in the basin in which Miocene units could not only distinguished by lateral and vertical facies changes but also clay mineral associations from Oligocene-Eocene units.