Abstract: The crustal movement observation network of China was set up in 1999.It has collected a long term GPS data set over three years which has made great contributions to crustal deformation study.We compute the GPS time series of all 25 permanent stations of CMONOC from March 1999 to March 2002 using GIPSY software of JPL.The average daily solutions of most GPS sites are over 850.The unfiltered GPS results are transferred to ITRF97 and then 10th-degree polynomialed for analysis.We can distinguish an annual period from the fitted curve of vertical time series.The crest and trough appearing at different season in different region indicates that the vertical accuracy of GPS measurements are mainly caused by atmosphere modeling error existing in the GPS computation.In order to study the crustal deformation,the GPS site must be established on stable bedrock.The repeated survey should be occupied at the same season and a long time series of GPS observations are necessary as well.