This study was performed to evaluate of temperature effects on motor unit action potentials by automatic method in twenty healthy adults. Twenty separate motor unit action potentials were recorded in the biceps brachii with concentric needle electrodes at the three different skin temperatures. The skin temperature of the arm was cooled with coldpack below 24oC and warmed with infrared above 38.5oC. The measured parameters of motor unit action potentials were amplitude, duration, area, spike duration, spike area and number of phases and turns. As the skin temperature was decreased, all parameters except number of turns were increased(p<0.05). And the larger changes were developed by cooling than warming. At three temperature settings, rise time was inverse relationship with amplitude and number of phases and turns. And in general other parameters of motor unit action potentials showed significant positive correlation. Therefore the temperature is a very important considering factor and must be standardized or controlied during analysis of motor unit action potentials.