We investigate the source process of the May 12, 2008 Wenchuan earthquake using teleseismic waveform data. We divide the source area into southwestern and northeastern segments and estimate the probable dip angle for the northeastern fault segment by analyzing the waveform misfit of the inversion. The results of the slip distribution indicate a thrust motion around the epicenter, while a strike–slip motion dominates from about 110 km northeast of the epicenter to the northeastern end of the fault. The maximum slip and total seismic moments are estimated as being up to 11.0 m and 1.2 × 1021 N m (Mw 8.0), respectively. Additionally, we simulate broadband seismic waves to verify our source model, assuming realistic three-dimensional earth structures and using the spectral-element method. The resulting synthetic waveforms agree well with the observed waveforms for both body and surface waves in the frequency range in the period from 20 to 150 s. In our model, comprising high-angle strike–slip mechanisms in the northeastern segment, the misfits of the surface wave in the range from 60 to 500 s is systematically smaller than the corresponding values in other source models comprising thrust or low-angle strike–slip mechanisms. The Coulomb stress distribution in the northeastern segment computed by using our model indicates positive and large changes induced by the rupture in the southwestern segment. We consider that the source fault of the 2008 Wenchuan earthquake is composed of at least two main fault segments: one with a low dip angle and the other with a high dip angle, which are dominated by thrust and strike–slip motions, respectively. We further consider that the rupture might transfer between these two faults during the mainshock.