It is disclosed that the flexible polyurethane foam prepared by reacting polyisocyanate with polyol and/or polymer polyol containing dispersed polymer microparticles obtained by polymerizing an unsaturated compound, water, catalyst, surfactant and, when needed, crosslinking agent and other auxiliary agents, whererin a portion or the whole of the polyol and/or the polymer polyol is prepared in the presence of a compound having a nitrogen-phosphorus double bond as a catalyst, have excellent wet compression set, fatigue by constant-load pounding and dynamic durability.