Interface morphology of strained In0.5Ga0.5As/GaAs wells grown by molecular beam epitaxy from 470 to 570 °C has been examined by photoluminescence at 77 K. Transition from the two‐dimensional to three‐dimensional (3D) growth was observed after a critical layer thickness, which largely depended on the growth temperature and was far ahead of the formation of misfit dislocations. Luminescence from the layer after the start of 3D growth had a high quantum efficiency but was rather broad or even split. The 3D growth started at nine monolayers for 470 °C and four monolayers for 540 °C. The reason for the onset of 3D growth is discussed in terms of In segregation.