In the process of measuring flatness, there are many parameters that affect accuracy of the measurement. This paper investigated temporal change in flatness of flat surface due to optical mounting and gravity. Flatness and topography of flat surface diameter range from 60 mm to 300 mm were recorded and evaluated by using Fizeau interferometer. Differences in mounting and geometry of the optical flat lead to difference in stabilization time. The experiments were carried out from the beginning of optical flat mounting vertically while the optical flats were initially kept in horizontal orientation. The temporal study was evaluated from 8 positions along X-axis and Y-axis. The experiment results show that with three-points mounting, stabilization time required is longer than the sling type. Moreover, large (heavy) optical flat also presented bigger temporal change which is not only come from size but also environmental condition.