Precipitable water vapor (PWV) is one of important green-house gases in the atmosphere. It affects the energy balance of the land-atmosphere system. The evolution of cloud and precipitation has a close relationship with the PWV. In the climate change, the PWV has a positive effect on temperature. Ground-based microwave radiometer can make all day unattended measurement of the PWV. Three different PWV measurements are compared. The PWV mean bias between the radiometer and the radiosonde is 0.281 cm. The PWV mean bias between the radiometer and the GPS is 0.322 cm. The PWV mean bias between the GPS and the radiosonde is 0.728 cm. In Beijing, there exist different diurnal variations in different seasons. The mean diurnal variation is about 0.162 cm in autumn, 0.130 cm in winter, 0.229 cm in spring, and 0.276 cm in summer. Moreover, the maximum PWV appears mostly at 0000 LST or 2300 LST in the four seasons. The minimum PWV is the same as the maximum PWV, except that the minimum PWV appears at 1000 LST in winter. The variation of PWV per hour decreases in the order of summer, autumn, spring and winter. Moreover, in every season, both percentages of the positive and negative PWV variations per hour are nearly 50%. The hourly correlations between the PWV and temperature are analyzed for the four seasons. The hourly correlation coefficients decreases in the order of autumn, spring, winter and summer. In autumn, spring and winter, all the hourly correlation coefficients are positive. However, in summer, the hourly coefficients are negative from 0900 LST to 2200 LST.