Abstract: A 10-year(1999-2008) half-hour dataset of precipitable water(PTV)vapor amountwas produced from ground-based gbbalpositioning system(GPS) measurements in the Lhasa river valley of the Tibetan Plateau.The diurnal variation of GPS-PW is investigated using the dataset aswell as hourly observations from automatic weather stadons in the valley.The result reveals a pronounced diurnal variation in GPS-PW，reaching the mininum around 08:00-10:00 UTC and the maxinum around 15:00-18:00 UTC during all seams.The average diurnal variations of GPS-PW are 1.0 mm，1.7 mm，1.0 mm and 0.8 mm in spring, summer, autumn and winter, respectively.The diurnal(24 h) and semidiumal(12 h) cycles of the GPS-PW were fitted using the harmonic analysis method.The resups indicate that diurnal and semidiumal cycles are the main signal of diurnal variation of GPS-PW during all seatens.The diurnal cycle of GPS-PW is the strongest in summer and the weakest in winter.The semidiumal cycle of GPS-PW is the strongest in summer and the weakest in spring.The relationship between diurnal variations of GPS-PW and that of rain fallwas conducted in summer.It can be seen that the occurrences of the diurnal variations of GPS-PW are nearly 2 h earlier than those of mean hourly p recip nation and p recip nation frequency in summer.The diurnal variations of GPS-PW have an important impacton the diurnal variations of hourly rainfall in summer.