In this paper is presented the possibility of. finding static reservoir pressure by extrapolating dPw/dt (rate of B.H.P. build-up) to zero, in case of compressible fluid flow. By plotting the Japanese oil field data on the bottom hole pressure build-up to Pw-dPw/dt diagram (see Figs 1, and 2), where Pw is bottom hole pressure, curves were obtained instead of straight line such as the relationship of incompressible fluid flow.