Compressional wave velocities (Vp) of gabbronorite, pyroxene granulite, wehrlite and websterite were measured up to 1000¿C at 1 GPa during heating and cooling. The wehrlite and websterite, which contain secondary hydrous minerals, show irreversible change in temperature derivative of Vp (∂Vp/∂T). During heating, these rocks show sudden decrease in Vp when dehydration reactions took place. During cooling, the rocks show a linear increase in Vp. In contrast, the gabbronorite and pyroxene granulite, which are nearly dry crystalline assemblage, show reversible and discontinuous change in ∂Vp/∂T at around 400¿C. It changes from -2.4 and -2.1 ¿ 10-4 km s-1 ¿C-1 to -4.7 and -4.5 ¿ 10-4 km s-1 ¿C-1. This reversible and discontinuous change in ∂Vp/∂T is difficult to explain by the previously proposed mechanisms, such as thermal cracking, dehydration reaction, and/or partial melting. The data suggest that this reversible and discontinuous change would be associated with phase transition in plagioclase.