Abstract: The methods of integrated water vapor estimates in the atmosphere using the GPS and the sun photometer are introduced. The atmospheric water vapor is able to affect the propagation of GPS signal. Therefore, the GPS can be used to estimate the integrated water vapor amount or precipitable water (PW). Water vapor can absorb solar radiation in the near infrared bands of 940 nm, which can directly affect the signals observed by the sun photometer. By applying the relation between atmospheric transmittance and water vapor amount, precipitable water in clear sky can be determined using measurements from the sun photometer. The comparison among the GPS, the CE318 sun photometer and the radiosonde for water vapor estimation is presented from May to July in 2008 at Xilinhaote station. The three methods are correlated well on describing water vapor trendency with correlation coefficient greater than 0.88, implying that both the GPS and the sun photometer can measure water vapor information in almost the same degree. But there is a difference in water vapor absolute values. Statistical analysis indicates that the GPSPW is 0.18 g/cm2 higher than the radiosonde and, the value of the sun photometer PW is lower than that of the radiosonde. The sun photometer is easy to carry and operate relatively. The GPS can be used under all weather conditions with no calibrations needed. The GPS appears to be used as a calibration tool for the parameters in the water vapor retrieval of the sun photometer.