The binary phase diagrams of several refractory metals such as Ta, W, Tc, and Re with three low melting metals Am, Cm, and Bk were calculated by the modified regular solution theory. Although there are several parameters that can influence the predictions of the phase diagrams, in this present study they were limited to the internal pressure difference. The internal pressure parameters of solid metals were calculated from the internal parameters of liquid metals. The phase boundaries are then calculated from the modified regular solution theory. A HP-41C program was applied to solve the analytical equations for solubilities and then the solubilities were transferred to analytical equation formats by the Chebyshev Polynomials program. Thus, the phase boundaries can be easily obtained as a function of temperature.