A comparative assessment is made of the observation data from the L band radar sounding system and 701 400M sounding system at Dazhou Station in Sichuan. The results indicate that the average temperature difference is -028 ℃; that below 200 hPa height is -005 ℃, and that above 200 hPa height is -052 ℃. The temperature data change resulted from sounding system change is obvious; 801% of temperature differences are between －10 ℃ and 10 ℃. The average geopotential height difference is -763 gpm; that below 100 hPa height is -113 gpm, and that above 100 hPa height is -1957 gpm. The geopotential height data change resulted from sounding system change is obvious; 890% of geopotential height difference is between -50 gpm and 10 gpm. The average relative humidity difference is -2%; 872% of relative humidity differences are between -20% and 10%. The average wind direction difference is 10°; 801% of the differences, are between -10° and 10°. The average wind velocity difference is 03 m/s; 95% of wind velocity differencs are between -5 m/s and 5 m/s. According to the average differences, the temperature, geopotential height, and relative humidity measurements from the L band sounding system are 〖JP2〗lower than those measured by the 701 400 M sounding system, but wind direction and velocity measurements from the L band sounding system are higher than those measured by the 701 400 M sounding system. The peak difference is large because of sounding system change. Owing to the differences of equipment, sounder manufacturing, measurement accuracy, sounder launching, message receiving, etc., the observational data are different, but influences on various elements are different.