Single quantum wells of GaAs with barriers of (AlAs)2(GaAs)2ultrathin‐layer superlattices have been fabricated and photoluminescence measurement was performed. The photoluminescence energy was found to change almost linearly against the well width and the photoluminescence linewidth to get smaller with decreasing well width, which is in marked contrast to the case of a usual alloy‐barriered single quantum well. It is indicated that the effective energy gap of the ultrathin‐layer superlattice barrier decreases with the decreasing well width in such a structure.