Abstract: The first-guess field (FG) has been used to assess the quality of sounding observation data in operation work for many years. A long term data analysis shows that the mean differences of temperature and geopotential height records between observations and FG at those neighboring stations using the same type of radiosondes are very small, while the differences for those neighboring stations using different types of radiosondes are significant. It can be inferred that FG can reflect the regional differences of meteorological elements caused by the synoptic situation. So it is feasible to compare neighboring stations records by using FG as a relative reference, which can eliminate the regional synoptic situation differences. Undoubtedly, such comparison should be applied in small region (i.e., between neighboring stations) by using synchronous observation records, and only sufficient (long term) sampling data has statistical representation.Hongkong sounding station has used the RS90/RS92 radiosonde manufactured by Vaisala since 2005. And during 2007—2009, the L-band radiosondes are used at several stations nearby Hongkong, such as Wuzhou, Shantou, Dongyuan and Yangjiang. So 5 stations above are appropriate for indirect comparison of L-band and RS92 radiosondes by using FG. It shows that there exist significant systematic differences between L-band and RS92 radiosonde systems, for both the temperature records and geopotential height records at each standard pressure level, through the two-year statistical comparison.At 19:00, the mean differences of temperature and geopotential height records are very small in the troposphere. In the stratosphere, the temperature measured by L-band radiosonde is lower than that measured by RS92, and the difference increases with height, reaching over 1℃ at 20 hPa height. The increasing temperature difference with height brings on an increasing geopotential height difference, which is over 30 gpm at 20 hPa height.At 07:00, the temperature measured by L-band is higher than that measured by RS92, which leads to a higher geopotential value. The temperature difference reaches 0.4℃ in the middle and upper troposphere, and the geopotential height difference is over 20 gpm at 100 hPa height. In the stratosphere, although the temperature measured by L-band is lower than that measured by RS92, and the difference also increases with the increase of height, the cumulative difference of temperature at 07:00 is less than that at 19:00. So the geopotential height difference between L-band and RS92 records is only about 10 gpm at 20 hPa height at 07:00.The long term direct comparison between different types of radiosondes is difficult to be carried out because of the high cost. Indirect comparison by using FG is not only economical but can reflect the real difference of operational records, so it is well worth to be widely applied.