GaN vertical trench-MOSFETs incorporating molecular beam epitaxy (MBE) regrown channel are developed and investigated. The channel regrowth by MBE prevents repassivation of the p-type GaN body while promising higher channel mobility. Two different designs of the lateral portion of the regrown channel are compared: without or with an n+-GaN buried layer. Without an n+ buried layer, a respectable 600-V breakdown voltage (BV) is measured in the absence of edge termination, indicating a decent critical field strength (>1.6 MV/cm) of the regrown channel. However, the ON-resistance is limited by the highly resistive lateral channel due to Mg incorporation. With an n+ buried layer, the limitation is removed. Excellent ON-current of 130 mA/mm and ON-resistivity of $6.4 ~\rm {m\Omega \cdot cm^{2}}$ are demonstrated. The BV is limited by high source–drain leakage current from the channel due to drain-induced barrier lowering (DIBL) effect. Device analysis together with TCAD simulations points out the major cause for the DIBL effect: the presence of interface charge beyond a critical value ( $\sim 6\times 10^{12}\,\,\rm {cm^{-2}}$ ) at the regrowth interface on etched sidewalls. This paper provides valuable insights into the design of GaN vertical trench-MOSFET with a regrown channel, where simultaneous achievement of low ON-resistivity and high BV is expected in devices with reduced interface charge density and improved channel design to eliminate DIBL.