One of the factors effecting accuracy of GPS measurement is observation period. To reach to desired accuracy, GPS observations are collected in a sufficient time of period. In other word, optimal measurement period is required to be decided. As the nature of GPS observations, more observation increases the cost of measurement process. In this study, relationship between observation period and accuracy of point positions in interest was investigated for the bases up to 20 km. For this purpose, in a network with 8 points and 19 baselines, GPS observation were collected for 253 minutes using static GPS observation technique. After ward, observations were divided in to 15, 20, 25, 30, 45, 60, 90, 120, 150, 180, 210, 240 and 253 minutes, than each one group of observation were post processed Leica Geo Office software. As out come, correlation coefficient describing the relationship between positional accuracy for each point and observation period were computed and then tested these coefficients for significantly.