This paper presents a step-by-step procedure to study the impact of each thermal phenomenon occurring in electric machines on their internal temperature distribution. First, the Monte-Carlo method is chosen as a global sensitivity analysis technique. This method is applied on a thermal network of a prototype consisting of a permanent magnet based integrated starter-generator machine (ISG). Accordingly, the variation of electric machine critical temperatures in windings and rotor’s permanent magnets is evaluated. In addition, a sensitivity index is defined for identifying network parameters that cause significant variation in the machine critical temperatures and should therefore be the focus of attention when designing electric machines. Finally, experimental tests are performed on the prototype and the difference between simulated and a measured temperature is presented.