Abstract: HY-1 Satellite was launched successfully on May 15th, 2002. It is the first ocean color observation satellite in China. On the other hand it is the first long life experimental and operational small satellite in China. The on-board GPS receiver was successfully used in navigation and give out the precise time. The positioning principle, flow chart and application of the GPS receiver on HY-1 satellite are introduced in this paper. A method to confirm and verify the on orbit positioning data is discussed. Using the Two Line Elements （TLE） from NASA web, the satellite orbit is calculated by the Satellite Tool Kit （STK） software so to obtain the nadir positions, then, the real GPS data at that time are processed and converted to the nadir positions. The nadir geodetic positions from GPS data are compared with those from the TLE. The difference of nadir positions between GPS and TLE methods are given. It is very helpful to give the comparison using the real on-board data.