High-speed direct modulation capability was investigated in 1.55-μm GaInAsP/InP distributed reflector (DR) lasers with wirelike active regions in terms of carrier transport from GaInAsP optical confinement layers (OCLs) to the active regions. Theoretical analysis revealed strong dependence of the modulation bandwidth on the thickness of the OCLs and width of the wirelike active regions. To confirm this prediction, two DR lasers with OCLs of different thicknesses (120 and 40 nm) were fabricated and their 3-dB bandwidths (f3 dB) under small-signal modulation were compared. The device with the narrower OCL exhibited f3 dB exceeding 15 GHz and clear eye opening under 25 Gb/s modulation, whereas that with the thicker OCL had f3 dB of only 2 GHz. These results were in good agreement with the theoretical predictions.