The migration of Ca ions during thermal decomposition of Ca-deficient hydroxyapatite (Ca-def HAp) involving the formation of β-TCP was discussed in the present paper. The migration of Ca ions at high temperature was examined by studying the thermal behavior of Sr ions in Sr-substituted Ca-def HAp using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (TEM-EDX). Substitutions of Ca ions in Ca-def HAp by Sr ion were carried out by hydrothermal treatment in Sr(NO₃)₂ solution. The composition of samples obtained by hydrothermal treatment was identified as Ca_9.27Sr_0.27(HPO₄)_0.46(PO₄)_5.54(OH)_1.54. In addition, it was considered that Sr ions would be in Ca(II) site in Sr-substituted Ca-def HAp lattice. Since the thermal behavior of Sr-substituted Ca-def HAp was almost the same as that of Ca-def HAp, the migration of Sr ions in Sr-substituted Ca-def HAp during thermal decomposition could be regarded as the same as that of Ca(II) ions in Ca-def HAp. By TEM and TEM-EDS analysis, it was revealed that Sr ions were detected only in β-TCP phase after thermal decomposition. Therefore, these results suggested that β-TCP phase would be formed by the priority migration of Ca(II) ions in Ca-def HAp.