We have demonstrated second‐harmonic generation in a GaAs/AlAs vertical cavity grown on a (311)B GaAs substrate. Second‐harmonic light of 492 nm was observed and its efficiency was measured to be 1.4×10−4%/W. The cavity confines high intensity of fundamental field, resulting in efficient second‐harmonic generation. Quasiphase matching was realized with the stacked GaAs/AlAs layers inside the cavity, which was designed taking into account the strong absorption of second‐harmonic power in GaAs layers. We show that conversion efficiency of the GaAs/AlAs vertical cavity could be more than 10%/W if the optimization is completed. ©1995 American Institute of Physics.