Abstract: The source parameters of aftershocks and the Q values of the crustal medium under the Tangshan aftershock area are estimated from respectively the half cycles and amplitudes of the first P arrivals of Tangshan aftershocks. The results obtained indicate that the stress drop gradually increases with distance from 2 bars at Tangshan and vicinity toward the northeast and southwest to a maximum of 34 bars. A tentative explanation has been proposed for the relation between the occurrence of the aftershocks and the distribution of quality factors of crustal medium.