The purpose of this paper is to provide a better understanding of the effects of the operating conditions on the performance of the high effeciency In P-IMPATT diodes. These effects are investigated using the full-scale computer simulation of these devices. The simulation takes fully into account all the physical effects pertinent to the IMPATT diode operation. It is found that by increasing the RF-voltage level across the diode terminals, three different modes of operation can be obtained successively. The first one is the conventional IMPATT mode. The second one is .a high-effeciency mode. In this mode, the avalanche-generated pulse (AGP) is bunched and accelerated during the second half cycle causing the induced current Ji to increase . In the third mode, which is also a high effeciency one,the AGP is prematurely collected. It is found that by increasing the operating frequency ,the premature collection of the AGP is either delayed or suppresed but the bunching and acceleration of this pulse become less pronounced. It is found that by increasing the DC-bias current, the premature collection of the AGP is either delayed or suppressed whereas the acceleration and bunching of this pulse are inhanced. On the other hand,the trapping of electrons behind the AGP becomes more significant. It is demonstrated that the optimum values of both the DC-bias current and the RF voltage level increase with the operating frequency. The possible applications• of the In P IMPATT diodes to airborne radar systems are indicated.