The temperature-pressure phase diagram of BaTiO3 has been determined by means of a dielectric measurement. All successive phase transition temperatures decrease with increasing pressure with critical index phi = 1, in the classical limit, and phi = 2, in the quantum limit. The dielectric behavior close to the quantum region agrees with the quantum theoretical prediction. In BaTiO3 the crossover region from the classical limit to the quantum one is observed in the range of pressure 4 to 8 GPa and temperature 0 to 200 K.