We found alternate oscillations in AlGaAs/GaAs single-quantum-well (SQW) quasi-stadium laser diodes (QSLD). The alternate oscillations in QSLD can be well explained by the interactions of two different cavity modes due to the geometrical symmetry of QSLD. Unlike the alternate oscillations in semiconductor ring lasers that need the waveguides to extract the light signals, the alternate oscillation signals in QSLD are obtained from the emitting lights spreading over the space. We also showed the alternate oscillation frequencies are inversely proportional to the amplitudes of two different cavity modes. This result is consistent with the theory of locking of two modes explaining the locking phenomenon between two nearly degenerated modes due to the nonlinear interactions in an active medium.