The temperature(T)-pressure(P) phase diagram of the diheptadecanoylphosphatidylcholine (DHPC) multilamellar vesicle was constructed by the method of high-pressure light transmittance. The DHPC bilayer membrane at ambient pressure undergoes the pretransition (at 42.9°C) from the lamellar gel to the ripple gel phase, and succeedingly the main transition (at 49.0°C) from the ripple gel to the liquid crystal phase. Slopes of the T-P diagram were 0.224 K MPa−1 for the main transition and 0.126 K MPa−1 for the pretransition, respectively. Interdigitation of DHPC bilayer membrane was observed at the high pressure beyond 80 MPa. © 1998, The Japan Society of High Pressure Science and Technology. All rights reserved.