Summary form only given. Rotating electrical machines are vital elements in the modern world, they generate the electrical energy used in all sectors of society, they also convert the electrical energy into mechanical power, to drive all kinds of processes where movement is required. Therefore, it is important to ensure that they operate efficiently and reliably. The lifetime of an rotating electrical machine is defined by the operation temperature of the insulation system, in this way, the temperature increase in insulation of electrical machines above stated limits, is considered as an important cause of deterioration and reducing the lifetime of the engine, therefore, in machines of great importance or laboratory tests, it is essential to have a thermal monitoring system. This paper deal with the design of a wireless system for on-line thermal monitoring that can be applied to any type of rotating electrical machines.