Abstract: The effects of bipolar flow on the performance of planar diode are analyzed and simulated by a PIC code. The effects of bipolar flow in an MILO device with radial emission are simulated. Some qualitative conclusions in this kind of microwave tube with low impedance are presented. The beam current increase caused by the bipolar flow in the tube may remove the operation point away from the optimal operation point. As a result the microwave output power and efficiency of the tube decrease. When the voltage gets higher, the work point of the tube is further away from the optimal operation point. The operation point of the tube would move below of B-H curve when the voltage is high enough. The tube is over-insulated and can not work well.