Abstract: To reveal the rolling contact fatigue behavior and failure mechanism of 18CrNiMo7-6 gear steel under different heat treatment states, this study conducted a comparative investigation on two states of 18CrNiMo7-6 gear steel, namely conventional carburizing heat treatment （CHT） and carburizing deep cryogenic treatment （DCT）. The relationship among microhardness gradient, residual stress evolution, and fatigue life under high contact stress （2 573-3 100 MPa） was analyzed. The experimental results show that under high load conditions, the contact fatigue life of DCT specimens is significantly lower than that of CHT specimens, with an average reduction of approximately 56.1%. Failure analysis indicates that the difference in fatigue performance arises from the synergistic effect of gradient microstructure and stress state. The CHT process constructs a gentle hardness gradient with a tough interior and a hard exterior, and maintains a stable bidirectional residual compressive stress field in both tangential and axial directions under cyclic loading. Meanwhile, the tough core promotes plastic blunting and coordinated deformation at the crack tip, leading to failure mainly in the form of progressive wedge spalling controlled by the main crack. The DCT process increases the overall hardness but reduces matrix toughness and introduces initial tangential tensile stress. Under high cyclic loading, the axial residual compressive stress tends to relax, and cracks accelerate propagation at local stress concentration points, ultimately exhibiting shallow brittle failure characteristics. This study confirms that under heavy load conditions, constructing a gradient microstructure with a good combination of strength and toughness and maintaining the stability of residual compressive stress can more effectively enhance the rolling contact fatigue performance of materials than simply pursuing high surface hardness.