This paper presents a new gear design method which is different from traditional method--the safety factor method, it is based on reliability theory. It is mainly used to study the gear tooth bending strength. A formula for gear reliability is presented, and the program can be used to calculate the reliability design information of cylindrical gears accurately and reliable. This paper presents a new gear design method which is different from the traditional method - the safety factor method, it is based on reliability theory. A formula for gear reliability is presented, and the program can be used to calculate the reliability design information of cylindrical gears accurately and reliable.