The electrothermal behavior of a 1200V-30A gate turn-off (GTO)thyristor operating in its electrical circuit has been studied using two-dimensional (2D) simulation tools. The program realized for this purpose uses a combination of Silvaco tools(5) and takes into account the thermal environment of the GTO. It was validated by direct experimental measurements. The studied GTO is a Silicon device with a symmetrical doping profile (N+PN-P+). It operates in a dc-dc converter at a 400 Hz commutation frequency, its conduction time is in the range of 0.05-0.25 ms. The thermal environment of the device is reproduced by including its thermal parameters and those of its package in the calculation, the simulation accounts, then, for all the thermal generation and conduction in the device. The simulation program was validated by comparing the electrical parameters (anode and gate currents, anode-cathode voltage, and dissipated power) of the GTO, obtained in the same operating conditions, by both simulation and direct measurements. The transient behavior of the GTO during its turn-off at a nanosecond scale was then obtained. We first obtained the 2D time-dependent distribution of most of the internal electrical parameters (carriers, current density,...); then we obtained the 2-D time-dependent distribution of temperature inside the GTO.