An improved method of stabilizing a polyoxymethylene copolymer, which comprises(1) heat-melting a crude copolymer of trioxane and a cyclic ether and/or a cyclic acetal to form a first stabilized copolymer having a decomposition ratio (D.sub.220.sup.60), determined when it is heated at 220.degree. C. for 60 minutes in vacuum, of up to 2% by weight, and then(2) bringing the first stabilized copolymer into contact with steam containing 0.01 to 5% by weight of a basic compound at a temperature of at least 100.degree. C. but not exceeding the melting point of the copolymer to form a second stabilized copolymer.