We have demonstrated the vertical GaN planar-gate MOSFETs fabricated by an ion implantation process.  The fabricated GaN vertical MOSFET shows a specific on-resistance of 2.78 mΩ cm² and a breakdown voltage of 1200 V, by applying a Mg and N sequential implantation to improve the breakdown voltage of the pn-junction and the control of the MOS channel characteristics on the p-type ion implanted layer.  Consequently, the vertical GaN planar-gate MOSFETs with high breakdown voltage and low on-resistance could be realized by ion implantation process.  On the other hand, there are still many challenges for realizing practical GaN vertical MOSFETs, so continuous development is necessary.