The GPS data at 4 sites of GPS network in the Yangtze delta is used to investigate the spatial and temporal distribution of Zenith Hydrostatic Delay(ZHD) and Zenith Wet Delay(ZWD) over the Meiyu period of 2002.Radiosonde observations from Baoshan site are used to estimate ZHD and ZWD to validate the GPS data,and the ZHD and ZWD estimated by MM5 24-hour simulation also compared with the GPS observations to validate the ability of the ZHD and ZWD prediction of MM5.Then,a method for correcting the simulation bias of ZHD of the model is introduced.The results show that GPS is an instrument for detecting atmospheric ZHD and ZWD with a detection accuracy comparable with radiosonde.The ZHD and ZWD from different GPS sites exhibit their individual features that ZHD has an obvious periodic change and ZWD is related to weather.The reanalysis fields of MM5 on the whole are able to reveal the distribution of ZHD,but usually has an underestimation of ZWD.The 24h-simulation of MM5 can show the change tendency of ZHD with a bias in depicting its diurnal varation.MM5 has also an ability of simulating ZWD on the whole with a bias of underestimation.The Zenith Total Delay(ZTD) is underestimated by MM5 simulation and the bias is around3cm,which can be reduced by means of correcting the surface pressure.