Results of a 1.5 MW, 110 GHz short (3 microsecond) pulse gyrotron experiment are reported. The gyrotron magnetron injection gun operated at full voltage (96 kV) and current (40 A), producing up to 1.4 MW at 110 GHz in the TE22,6 mode. The operation of the TE22,6 mode, as well as nearby modes, was measured as a function of magnetic field at the cavity and at the electron gun to produce a mode map. Significant mode competition was found, but the measured efficiency of 37 % in the TE22,6 mode, without a depressed collector, is close to the design value of 39 %. The beam alpha, the ratio of transverse to axial velocity in the electron beam, was measured with a probe. The alpha value was found to be 1.33 when the gyrotron was operating at conditions for achieving the highest output power level (1.4 MW.) This value of alpha is less than the design value of 1.4, possibly accounting for the slightly reduced experimental efficiency. The output power and efficiency, as a function of magnetic field, beam voltage, and beam current, are in good agreement with nonlinear theory and simulations with the MAGY code. These results are promising for the development of an industrial version of this gyrotron capable of long pulse or CW operation.