Abstract: A new type of magnetically insulated transmission line oscillator (MILO)-planar MILO is put forward to solve the low frequency and low efficiency problems of MILO. The new device can be expanded along the x direction of the plane and it is also a microwave tube with low impedance and high power. The planar MILO uses a low external magnetic field to take the place of conventional magnetic insulation magnetic field so that we can enhance the efficiency of the microwave output. Through numerical calculation, the impacts of several structure parameters of the planar MILO coaxial SWS with plane-wrinkle surface on its high frequency characteristic are analyzed. Then the external magnetic is further optimized with 2.5 dimensional particle-in-cell simulation. High power microwaves generated from the planar MILO have the average output power of 1.22 GW, central frequency of 6.56 GHz and efficiency of 30%, when the beam voltage is 800 kV.