Polyunsaturated phospholipids interact complexly with other membrane components. We have examined pair interactions among ternary lipid bilayers composed of saturated DPPC, polyunsaturated PLePC, and cholesterol in the liquid-ordered and the liquid-disordered phases by all-atom molecular dynamics simulations. The results show that PLePC exhibits strong repulsion with DPPC and cholesterol in the liquid-disordered phase. When the bilayer changes to the liquid-ordered phase, the repulsion of PLePC with DPPC and cholesterol reduces significantly. The phase state of the bilayer which affects the order of acyl tails as well as their density distributions along the bilayer normal is a key factor regulating the role of PLePC in lipid mixtures. Polyunsaturated phospholipids play a strong repulsive role in the liquid-disordered phase but a weak role in the liquid-ordered phase.