A flexible polyurethane foam which has a low density, enhanced impact resilience, improved compression set and excellent effect on the reduction of 6 Hz vibration transmissibility and is obtained by a reaction of an organic polyisocyanate composition, that is, a mixture of (a) polymethylenepolyphenyl isocyanate containing from more than 60 to 90% by weight of a two benzene ring compound and having a weight ratio [three benzene ring compound]/[four or more benzene ring compounds + less active ingredient] of 1.1-20.0, and (b) tolylene diisocyanate in a ratio (a)/(b) of 97.apprxeq.80/3.apprxeq.20 by weight, or by reaction of a prepolymer derived from said organic polyisocyanate composition and a specific polyol, with a resin premix comprising a water blowing agent and an active hydrogen containing compound; and a process for preparing the flexible polyurethane foam.