The coaxial virtual cathode oscillator (vircator) at Texas Tech University was studied by varying the pulse length of the electron beam. Previous experimental results obtained with electron beam pulse width of 25 ns have indicated that the microwave output power had not saturated before the electron beam power terminated. Therefore, it has been expected that the maximum microwave power can be increased if the electron beam pulse is lengthened. Experiments were carried out with the electron-beam pulse length of 25 ns and 70 ns, respectively, while keeping other parameters the same. The microwave outputs obtained with different electron-beam pulse lengths are compared in order to understand the effect of the pulse length on the microwave efficiency of the coaxial vircator.