This study investigates the contact stress of concave conical involute gear pairs with small intersected angles by means of finite element analysis. The complete mathematical models of the gear pair, including the working tooth surfaces and the fillets have been derived based on the theory of gearing and the generation mechanism. The commercial software, ABAQUS, capable of contact analysis of two 3-D deformable bodies is applied to evaluate the stress distribution on the tooth surfaces. Several numerical examples are presented to demonstrate the stress analyses of the gear pairs with various gear design parameters. The results show that concave involute gear pairs relieve the high contact stress problem which is inherent in conventional conical involute gear pairs.