The focal mechanisms of the 1991 Chiali earthquake sequence were investigated by considering the first motion data and available strong motion waveforms. Due to the sparse distribution of the first-motion data, the first-motion solutions are multiple for the earthquake sequence. By considering the waveform mechanism, which is determined by considering the amplitude ratio of P, SV and SH waveform, as the constraint, we chose the optimum first-motion mechanisms of the earthquake sequence. Either the optimum first-motion mechanisms or the waveform mechanisms of most of the aftershocks show thrust faulting mechanism. Except for the event E-3, the waveform mechanisms of the earthquake sequence have a northeast-southwest striking plane similar to the distribution of aftershock sequence, while the optimum first-motion mechanisms of the earthquake sequence show more complex focal mechanisms. Compared with the aftershock distribution in consideration of the waveform mechanisms of the earthquakes, the fault plane with NE-SW strike and NW dipping plane is suspected as the preferred fault plane. Since the location of the earthquake is near the transition between the extension mechanism of southwestern Asian passive margin and the compression mechanism due to Luzon arc collision, the thrust faulting mechanism of the Chiali earthquake might indicate an ongoing compression mechanism due to arc-continental collision in such an extensional environment. The b value of about 0.7 in this region implies a relatively stable tectonic environment compared with other regions in Taiwan. The larger ratio of the logarithm of cumulative seismic moment of aftershocks to that of the seismic moment of the mainshock for the 1991 Chiali earthquake (about 1) also suggests a smaller stress drop for this earthquake is compared with other moderate size earthquakes in the Chianan region.