Plastic forming for magnesium alloy requires warm working. However, warm working causes micro-structure change or strength decreasing to mechanical characteristics grant material by work hardened, or grain refinement. To study warm working conditions, it is important to find out β phase (β-Mg₁₇Al₁₂) transition of magnesium alloy and crystal grain growth in heating process. In this report, rolled AZ61 magnesium alloy crystalline structure changing in heating process were in-situ measured by synchrotron radiation. As results, crystal grain growth of α-magnesium, and β-Mg₁₇Al₁₂ solution was not found under holding temperature 400℃ till 50 seconds. Furthermore, temperature rising rate 　2℃/sec or over is necessary at time.