Abstract: An S-band tapered magnetically insulated transmission line oscillator with slowly tapered vanes and sharply tapered vanes has been studied at long-pulse operation. Under the diode voltage of 350 kV, the diode current of 25 kA and the pulse duration of 128 ns, the output microwave with the power of 500 MW and the pulse duration of 90 ns has been obtained in the experiments. Further analysis showed that the poor microwave shape attributed to the poor diode voltage waveform and better diode voltage waveform was crucial to achieving a longer microwave pulse. In addition, two kinds of load structure were used to collect the e-beams. The experimental results indicat that using the graphite load could get the better performance than the stainless steel load.