Abstract: The evolution of pores and fractures, the maturation of organic matter and the development of original formation pressure (OFP) were considered in a contect, and the spatial and temproal relationships among these three aspects were discussed in this paper. It was proposed that the development and evolution of OFP could be divided into two periods, i.e. the earlier period (pri-Himalayan) and the later period (since-Himalayan). Athough there were many factors contributing to OFP during the earlier period, the hydrocarbon generation was the most important mechanism and OFP increased progressively. Accordingly, the increment of OFP in each stage was calculated. In the later.period,the great lateral squeering force in Himalayan strongly affected OFP formed in the earlier period.Therefore, the Variation of OFP by structural squeeze was semi-quantitatively represented through modelling on tectonic stress field, and the difference of the present OFP in area could be directly related to the extent of tectonic activity in Himalayan.