This paper proposes a new self-consistent non-linear operation analysis of the gyrotron which considers the oscillation build-up process. In this method, completely relativistic electron equations of motion and wave equations for open-end cavity with an electron beam current source are solved independently by the iteration procedure so that the oscillation build-up process from "rising" until reaching "steady-state" may be considered. Analyzing the operation of a 35GHz TE
011mode gyrotron, this paper presents a comparison with the results of conventional methods for the efficiency and oscillation frequency, together with an analysis of the oscillation build-up process. By using the new method, efficiency is enhanced to 47% with a low-Q cavity Qext=215 at beam voltage 70 kV and

is 1.5.