The use of co-polyesters with a low melting point in the textile industry has received much attention in order to use the ability of strength by melting in different forms, including fibers, yarns, fabrics and non-wovens. One of the important indicators of molecular weight and flowability of fibrous polymers before their processing is to evaluate the melt flow index. With this simple method, the flow behavior of fibrous polymers can be investigated at low cost and in a short period of time before the melt-spinning process. In this research, the value of melt flow index of four different grades of copolyester with a temperature of 198 °C was determined. Therefore, checking the flow behavior by using the melt flow index measuring device, temperature changes, residence time and load weights are used. The results showed that intrinsic viscosity is one of the influencing factors on the value of melt flow index and flow stability in different test conditions, and changes in temperature and residence time have the greatest effect on the change of melt flow index. As the temperature and residence time increase, the viscosity decreases. Also, by carefully examining their melt flow behavior, it was observed that among the four different grades of copolyester examined, the grade with intrinsic viscosity of 0.66 and 0.69 dl/g is the most suitable option for the melt spinning process.