The dependence of lasing characteristics on stripe width in GaInAsP/InP double‐heterostructure planar stripe lasers, prepared by using sputtered SiO2film as a Zn‐diffusion mask, was investigated. The lasers of 10‐, 15‐, and 20‐&mgr;m‐wide stripes operated in a fundamental‐transverse mode with more than 20% differential quantum efficiencies per facet, while the 5‐&mgr;m‐wide stripe lasers operated in a multi‐transverse‐mode even just above threshold. The threshold current of 5‐&mgr;m‐wide stripe lasers increased greatly, and the lasing wavelengths shifted to shorter than those of stripe lasers with larger stripe widths. The 15‐&mgr;m‐wide stripe lasers showed good mode characteristics to operate in a fundamental‐transverse mode up to the output power of 22 mW per facet and in a single longitudinal mode over a wide range of currents. The cause of this dependence of lasing characteristics on stripe width was discussed, and it was assumed that the Zn diffusion into the stripe region using a sputtered SiO2mask is most responsible for the dependence.