We have experimentally determined the P-wave velocities of 68 typical UHP metamorphic rock samples among which 31 were collected from the CCSD (Chinese Continental Scientific Drilling) main hole and 37 from the surface outcrops in the Sulu orogenic belt (China) at ambient temperature and hydrostatic confining pressures up to 800 MPa. The velocity-pressure curves can be well described by a simple expression: V(P)=V_0+DP-B_0exp(-kP), where V_0 is the projected velocity at zero pressure if both porosity and cracks were absent; D is the intrinsic pressure derivative of velocity in the linear elastic regime; B_0 is the initial velocity drop caused by the presence of pores and cracks at zero pressure; and k is the decay constant of the velocity drop, which controls the shape of the velocity-pressure curves in the nonlinear poro-elastic regime. The above equation was used by previous workers as an empirical formula to describe the relationship between seismic velocity and confining pressure. Here we show that the equation can be directly derived on a rather general theoretical basis. The derivation has provided a clarification for the physical meaning of each parameter in the equation. It is suggested that henceforth the seismic velocities of rocks should be published in terms of these four parameters rather using tedious tables to list each velocity at each pressure for each sample as done before. The statistical properties of P-wave velocities in the Sulu UHP rocks provide a complete set of basic information for the interpretation of field seismic data from the root zones of continental convergent orogenic belts and modern and ancient subduction zones.