Acetal gears are the most widely applied type among these polymer gears. Acetal gears may suffer from many failure forms such as severe wear, tooth breakage or tooth surface cracks depending on the loading conditions and the lubrication and temperature environment. The contact fatigue performance of acetal gear relates to many factors including temperature and load, however, their influences on the mechanical properties and fatigue parameters of the acetal material are still not fully understood. In this work, the thermo-elastic-plastic constitutive equations are derived to describe the acetal material behavior during gear meshing process, and the modified Brown-Miller multi-axial fatigue criterion is proposed to estimate the polymer gear contact fatigue life. Durability tests of acetal gear against steel gear are conducted based upon a developed test rig. Results reveal that the temperature rise could significantly reduce the contact pressure of acetal gear due to the decay of the modulus, but still jeopardize the fatigue life on tooth flank because the fatigue properties are also affected by the temperature rise. The predicted location of fatigue failure zone on the tooth and the lifespan agree well with experimental observation. •Temperature effect on fatigue life of acetal gear is considered.•Thermo-elastic-plastic constitutive equations of acetal gear is derived.•The predicted fatigue lifespan agree well with test results.