In this paper, a time series of Precipitable Water Vapor (PWV) in 2 hour intervals for year 2005 has been estimated for Tehran region using observations of National Cartographic Center (Iran) permanent GPS station. Zenith Total Delay (ZTD) which is obtained from analysis of GPS observations is subtracted from Zenith Hydrostatic Delay (ZHD) that is determined from surface pressure, temperature and humidity. By applying transformation factor on the result of subtraction of ZTD and ZHD, namely Zenith Wet Delay (ZWD), PWV was provided. The results show that the maximum and minimum PWV occur in July and February with monthly average of 23.3 and 8.2 millimeters respectively. Also maximum variation during one month occurs in August with standard deviation of 6.5 millimeters. Good coherence between our results and UNB3m model validates estimated PWV