Abstract: The functiong of gates in gate associated transistor (GAT) are discussed and the relation between breakdown voltage and parameters of gate is studied using numerical method. The software used here is PISCES. The following conclusions are reached from calculation The improvement of breakdown voltage is notable because of introducing gate structure into a conventional transistor, The higher impurity concentrations in gate areas, or the deeper depth of gates, the higher breakdown voltage can be achieved, The spacing between gates is a delicate parameter which can be optimized to get a maximum value of BV_CEO.