A thermoplastic water-curable composition which is excellent in mechanical properties, heat resistance, dimensional stability, and moldability and from which a mechanical part of a complicated shape can be produced: a molding formed from the thermoplastic water-curable composition; and a process for producing the molding. The thermoplastic water-curable composition comprises: a thermoplastic resin which upon contact with water undergoes a cross-linking reaction and thus comes to have a high molecular weight, desirably one having silane groups in inner parts thereof and a water-curable composition, the amount of water-curable composition being desirably 200 to 700 parts by weight per 100 parts by weight of the resin The molding is formed from the thermoplastic water-curable composition. The process comprises molding the thermoplastic water-curable composition into a given shape and then bringing the molding into contact with water to age and cure it.