The bilayer phase transitions of saturated ether-linked phospholipids with different-sized polar head groups, dihexadecylphosphatidylethanolamine (DHPE), dihexadecylphos- phatidyl-N,N-dimethylethanolamine (DHMe₂PE) and dihexadecylphosphatidylcholine (DHPC), were observed by means of differential scanning calorimetry (DSC) and high-pressure light-transmittance. The temperatures of so-called main transition from the lamellar gel (L_β) or ripple gel (P_β') phase to the liquid crystalline (L_α) phase were almost linearly elevated by applying pressure. The slope of the temperature-pressure boundary (dT/dP) was in the range of 0.232 - 0.244 K MPa^-1. The main transition temperatures of these lipid bilayers decreased with increasing size of head group. On the other hand, there was no significant difference in thermodynamic quantities of the main transition among these lipid bilayers. Bilayer membranes of DHPE and DHMe₂PE underwent only a main transition from the L_β phase to the L_α phase, whereas the DHPC bilayer exhibited two phase transitions: the pretransition from the interdigitated gel (L_βI) phase to the P_β' phase and sequentially the main transition from the P_β' phase to the L_α phase. In other words, the interdigitation of DHPE and DHMe₂PE bilayers was not observed. It is well known that the interdigitation of ester-linked phosphatidylcholine bilayer, for example dipalmitoylphosphatidylcholine (DPPC) bilayer, is induced by hydrostatic pressure. Present results revealed a necessarily important condition for bilayer interdititation: lipid molecules have to include in structure a bulky head group of choline rather than the ester- or ether-linkage. A probable mechanism for pressure-induced interdigitation of bilayer is proposed.