The work aims to study the change of microstructure of carburized layer and matrix and its effect on contact fatigue performance after Nb microalloyed, so as to achieve long life contact fatigue life of gear. Carburizing heat treatment were carried out on gear steel 18CrNiMo7-6 with and without Nb microalloyed by means of vacuum carburizing furnace, contact fatigue tests were carried out by means of the rolling contact fatigue tester. Scanning electron microscope (SEM), X-ray diffraction (XRD), transmission electron microscope (TEM), electron backscattering diffraction microscope (EBSD) and rockwell hardness tester were used to detect the microstructure and hardness of the samples, and the influence of contact fatigue property was discussed. The results show that the surface was acicular martensite, residual austenite and carbide, and the matrix was lath martensite after carburizing heat treatment. The microstructure of the carburized layer after Nb microalloyed was refined, the dislocation density increased from 7.52×1015m?2 to 8.75×1015m?2, the content of residual austenite decreased from 23.6% to 15.4%, the hardness of the carburized layer increased from 58.6HRC to 59.4HRC, and the grain size of the core austenite decreased from 20.5 μm to 16.3 μm. Typical failure mode was spalling pit on contact surface. The spalling pit was rough and layered; The work hardening of spalling pits occured under the action of rolling contact stress, and the work hardening hardness of both Nb microalloyed and non-Nb microalloyed pits increases by about 1HRC, and the deformation resistance of spalling pits had little difference. For Nb microalloyed gear steel, fatigue lives L10=3.2×107, and L50=8.2×107; while for the gear steel without Nb microalloyed L10=2.0×107, L50=6.4×107. So after Nb microalloyed, the microstructure of the carburized layer was refined, and the dislocation density increased, which significantly inhibited the initiation of cracks. The hardness of the carburized layer increased slightly, and the rolling contact fatigue life L10 and L50 increased by 37.5% and 22%, respectively.