The contact ratio is an important performance index of gear transmission,but there is no computational formula of contact ratio for complex tooth surface transmission,such as face gears.The definition of face gear contact ratio is given,and the method of time difference which can be used to calculate face gear contact ratio based on the finite element gear tooth contact analysis is proposed.The contact ratio of loaded and unloaded modification face gear and standard face gear are calculated.The calculation results show that:1)the contact ratio of modification face gears reduce greatly compared with the standard face gears,2)the contact ratio of loaded modified face gears increase greatly compared with unloaded gears.A method to calculate face gear contact ratio is proposed.