P- (Vp) and S-wave (Vs) velocities of polycrystalline plagioclase (An51¿1) and gabbronorite including plagioclase (An49¿2) (62.3vol.%) were measured up to 700¿C at 0.6, 0.8, and 1.0GPa during heating and cooling. Both polycrystalline plagioclase and gabbronorite show reversible and discontinuous change in temperature derivative of Vp and Vs, ∂Vp/∂T and ∂Vs/∂T, respectively, at about 400¿C. The ∂Vp/∂T and ∂Vs/∂T of the polycrystalline plagioclase changes from -0.8 ¿ 10-4 to -3.4 ¿ 10-4 km s-1¿C-1 and from -1.1 ¿ 10-4 to -3.3 ¿ 10-4 km s-1¿C-1, respectively. The ∂Vp/∂T and ∂Vs/∂T of the gabbronorite varies from -1.0 ¿ 10-4 to -3.4 ¿ 10-4 km s-1¿C-1 and from -0.4 ¿ 10-4 to -3.5 ¿ 10-4 km s-1¿C-1, respectively. The reversible and discontinuous changes in ∂Vp/∂T and ∂Vs/∂T are attributed to a phase transformation of plagioclase. The present data suggest that the sharp decrease in Vp and Vs would cause low velocity anomaly under dry and subsolidus melt-absent conditions in the mid-to-lower crust of relatively high heat flow regions.