The hydrostatic-pressure-induced transition phase of BaS from the NaCl-type structure (B1) to the CsCl-type structure (B2) is investigated by ab initio plane-wave pseudopotential density functional theory method. It is found that the transition pressure from B1 to B2 phases is 8.2 GPa according to the usual condition of equal enthalpy. Through the quasi-harmonic Debye model, the dependences of the relative volume V/V0 on the pressure P, the thermal expansion parameter ratio (γ − γ0)/γ0 on pressure P, and the Debye temperature Θ and heat capacity CV on pressure P and temperature T are estimated.