In liquid filling systems using a cylinder with a reciprocating piston driven by pneumatic cylinder, In the filling stroke there are a lot of parameters affecting these systems and their performances ,one of these parameters is a piston stroke length. In this paper the influences of a piston stroke length and their parameters on the velocity of the piston has been studied in order to reach the optimum piston stroke length. First the mathematical module of velocity of the piston and the length of the stroke and other parameters is extracted, then experiments are done using a special testing stand equipped by all control and measuring tools. All reading data are converted into graphs. The results show that In liquid filling systems there is an ideal value range for the piston stroke length.