Abstract: Using the data of the earthquakes of magnitudes 3.5 to 5.0 occurred in the Yanyuan region, Yunnan Province in recent years, the Doppler Effect as shown by the first half period of the P waves from those earthquakes has been based upon to determine the rupture surfaces and the propagation velocities of the ruptures. The results show that the rupture surfaces coincide quite well with the regional tectonic lines.From our analysis, we can see that it would be better to use the unilateral rupture model than the bilateral one for describing the ruptures of the small and medium size earthquakes studied. Basing on the above results the source dimensions of these earthquakes were estimated.