One-pot polymerization of polyimide from 3,3’,4,4’-biphenyl tetracarboxylic dianhydride (BPDA) and 4,4’-oxydianiline (ODA) was examined. The equilibrium in the polyimide with water was examined in detail in p-chlorophenol solution during the polymerization. The equilibrium constant was expressed by log K = 1.50 + 1433 (1/T). The polymerization reaction is exothermic. The molecular weight increased with decrease of temperature.