Disordering of ZnSe/ZnS strained‐layer superlattices(SLSs) induced by Si ion implantation and subsequent low‐temperature thermal annealing was confirmed. Si ions were implanted (100 keV, 1×1016 ions/cm2 ) into SLSs (140 Å ZnSe‐140 Å ZnS, ten periods). By secondary‐ion mass spectrometry analyses, periodic profiles of Se and S were clearly observed just after the ion implantation; however, they disappeared after subsequent thermal annealing (450 °C, 3 h). Photoluminescence measurements showed the peak of ZnSx Se1−x alloyed crystal after thermal annealing. The disordering is mainly induced by the diffusion of defects generated by the ion implantation at the early stage of low‐temperature thermal annealing. This low temperature and planar process will be very useful to the fabrication of II‐VI compound semiconductor optical and electrical devices.