In this paper, a mechanism of the frequency variation during a pulse in TRAPATT oscillators and some methods for suppressing the frequency variation are demonstrated theoretically and experimentally. A model for explaining the frequency variation during a pulse is proposed, in which the avalanche delay time varies with the temperature variation so that the frequency varies. It is shown that this model is reasonable to explain the frequency variation during a pulse by a comparison between the frequency variation estimated from the model and directly measured one. It is shown that the model can also well explain the suppression mechanism how the frequency variation can be suppressed by the use of distorted bias pulse. This suppression phenomena have been already confirmed experimentally and reported by the authors. Moreover, possibilities of suppressing the frequency variation by controlling diode parameters are discussed. It is shown that the frequency variation can be suppressed by emphasizing a temperature depenedence or a time dependence of diode parameters causing an avalanche, such as the reverse current.