The phase behaviour of the discotic mesogen 5,10,15,20-tetrakis(4-n-dodecylphenyl) porphyrin(C12TPP) was investigated under hydrostatic pressures up to 350 MPa by high-pressure DTA and wide-angle X-ray diffraction measurements. Seven crystal polymorphs are found, depending upon the temperature and pressure. The discotic lamellar(D-L) phase is observed enantiotropically under pressures up to ca. 240 MPa, while the monotropic D-L phase appears on cooling under pressures between 240 and 310 MPa. Further increasing pressure above 310 MPa disappears completely the D-L phase, showing the crystal-isotropic transition.