This paper presents a new method to design and to determine the inertial moment of the fly wheel, according to the allowed maximum and minimum angular velocity. The advantage of this method is: in the same time of the dynamic analysis of the the mechanics we can fulfill the design of the fly wheel. With this method, the inertial moment of fly wheel is determined precisely. In the traditional method, first we have to determine the inertial moment of fly wheel, then, make static-dynamic analysis of the mechanism and find out the react force of the connect pair. For example, after work, one first goes home and then has to go shopping. But if we choose not to go home directly, while on the way home in passing do some shopping then go home. Even though it is not the shortest way, but in total we can shave time and rod. It is the same occasion with our design method too. Even though the process of calculation is more complicated than the formal method, but we can determine the reaction on the connect pair at the same time. So that in total decreased the amount of design work.