We studied the formation mechanism of Al-segregated regions in InAlAs/(1 1 0) InP by transmission electron microscopy (TEM). It is found that the Al-segregated regions observed as dark bands in the TEM image thread upward at certain intervals. The intervals of the regions agree well with the spaces estimated from the distances between one-monolayer steps (1-MLSs) on the substrate surface. This suggests that the 1-MLSs play an important role for nucleation of the Al-segregated region. Based on this finding, we also propose a model for the nucleation mechanism.