We investigated the dynamic characteristics related to the transient spectra of two types of transverse‐mode stabilized GaAlAs double‐heterostructure lasers: internally‐striped‐planar (ISP) and channeled‐substrate‐planar (CSP) lasers. These lasers exhibited a linear light‐current relation, stable transverse‐mode oscillation, and dynamic response with suppressed relaxation oscillations. Several differences in their spectral behavior have been observed. The dc spectrum of the CSP laser shows a single longitudinal mode, whereas the dc spectrum of the ISP laser shows multilongitudinal modes. In the ISP laser, the transient spectrum under fast pulse modulation is nearly identical to its dc spectrum. The CSP laser shows a transient spectrum that is broader than that in cw operation. The experimental results may be explained from the differences of their device structure and waveguiding mechanisms.