Isometric force response curve by electrically stimulated skeletal muscle contraction of twitch is important in order to judge the ability of the skeletal muscle and to distinguish disease state. In this report, we introduce a new mathematical model of the isometric force response curve, in order to estimate the muscle contraction behavior accurately. The new mathematical model of force response curve was expressed by a difference of fundamental function which was expressed by products of a step response function and a logistic function. The mathematical model was applied to the force response curve by electrically stimulated muscle contraction of twitch, using the rat gastrocnemius muscle (GC), intermediate vastus muscle (VI) and soleus muscle (SOL) . The results indicate new mathematical model showed better agreement with the force response curve of twitch than previous mathematical model in the rat skeletal muscle which has different muscle fiber type only changes the model parameters.