Based on the experiment records of L band radiosonde sounding conducted at Nanjing Station in January 2009, taking the altitude data measured by GPS as the standards, the radiosonde height and radar altitude are analyzed in detail. It is concluded that the difference between radiosonde height and GPS altitude increases with the increasing height. The overall height difference tends to increase with height and the amplitude of the difference fluctuates dramatically. Below 100 hPa, the reliability of radiosonde height is greater than that of radar altitude, which is the best choice; however, above 100 hPa, the reliability of radar altitude is better, though the stability is poor.