Compressional (vp) and shear (vs) wave velocities of plagioclase (An51±1) were measured up to 900 °C at 1 GPa. The temperature derivatives of vp (∂vp/∂T) and vs (∂vp/∂T) show a discontinuous change at ~400 °C. The ∂vp/∂T is −0.9 × 10−4 km/s/°C below 400 °C and −4.4 × 10−4 km/s/°C above 400 °C. The ∂vp/∂T also increases from −0.7 × 10−4 to −4.1 × 10−4 km/s/°C. These vp and vs show reversible changes between 25 and 700 °C. In contrast, both vp and vs increase (0.08 and 0.08 km/s, respectively) at 700–800 °C, and show irreversible changes after heating to 800 and 900 °C. The X-ray powder diffraction analysis shows that the run product heated to 900 °C shows a higher lattice angle γ than the run products obtained on heating up to 700 °C, which is comparable to the lattice angle γ of high and low plagioclase, respectively. We ascribe the discontinuous change in vp, vs, ∂vp/∂T, and ∂vs/∂T to the order-disorder transition of plagioclase at high temperatures.