In the present paper, thermal analysis of used spiral bevel gears in main gearbox of helicopter- belong to Iran Aircraft Manufacturing- is investigated. Firstly, with introducing the geometry properties of gears, basic lubrication and thermal analyses are considered based on standards of gear design such as AGMA. Then, in order to create the finite element model, initial and boundary conditions with considering the oil viscosity and calculating the friction coefficient, convection and heat conduction coefficients are determined based on experimental and analytical models in spiral bevel gear. It is noted that, the goal of finite element model is considered to reduce the complex calculation errors and increase the speed of problem solving. Effects of various parameters such as increasing the FLASH temperature and influences of initial temperature on it, contact stresses and heat fluxes, comparison of different mineral oils on the decreasing of temperature and fatigue life are examined. The obtained results of present work show that the FLASH temperature of main gearbox is linear function of initial temperature, so that FLASH temperature increases 56 centigrade in comparison of initial temperature. Also, it is demonstrated that the presence of various mineral oils in this system lead to reduce the solid-solid surface contact and friction coefficient. Moreover, these lubricants cause the cooling in the gearbox and enhancing more temperature, thus the employing these lubricants lead to exceed the system temperature to 90 centigrade.