DH lasers are believed to be one of the most suitable light sources for optical fiber transmission. However, the light output of DH lasers has usually such detrimental effects as a relaxation oscillation and a spectral broadening during pulse response. For high bit rate and long-distance fiber transmission, lasers need to have a high performance pulse responce and a narrow spectral width. We examined the dynamic characteristics of GaAs/GaAlAs DH lasers, which had an internally striped planar (ISP) structure using sulfur diffusion, by changing the stripe width. By narrowing the stripe width to the range of 5 to 7 µm of ISP lasers with an undoped active region and the cavity length of about 150 µm, significant improvement was achieved in frequency response, pulse response and spectral behavior as well. The frequency response was flat up to 2 GHz without a resonant-like peak. The relaxation oscillation was suppressed up to 10-mW light output. The spectral width at 400-Mb/s modulation was controlled within 20 without degradation of extinction ratio. Threshold currents were in the range of 55 to 70 mA. The light output increased linearly up to 10 mW without a kink. The external differential quantum efficiency was about 60 %. These lasers are operating stably after more than 1,000 hours at 70 under 3-mW light output. A narrow stripe-geometry ISP laser has a promising feature in that it can be used in high bit rate and long-distance fiber transmission.