The antiferromagnetic phase transitions in a CuO single crystal are studied by specific heat in magnetic fields up to 6T. The magnetic field dependence of the incommensurate-to-commensurate-antiferromagnetic transition atTL is found to be highly anisotropic.TL is observed to increase nonlinearly for Ba ∥c-axis, whereas, a linear reduction is observed forBa ∥b-axis. The magnetic field dependence ofTL and the jumps in magnetic susceptibility atTL are explained thermodynamically using the Clausius-Clapeyron equation.