Tunable electrical phase transitions based on the structural and quantum-state phase transitions in two-dimensional transition-metal dichalcogenides have attracted attention in both semiconducting electronics and quantum electronics applications. Here, we report gate-voltage-induced reversible electrical phase transitions in Mo0.67W0.33Se2 (MoWSe) field-effect transistors prepared on SiO2/Si substrates. In gate-induced depletion regions of the 2H phase, an electrical current resumes flow at 150 K < T < 200 K with decreasing T irrespective of the layer number (n) for MoWSe when n < 20. The newly appearing electron-doped-type conducting channel again enters the 2H-phase region when the back-gate voltage increases, accompanied by the negative differential transconductance for four-layer and monolayer devices or by a deflection point in the transfer curves for a multilayer device. The thermal activation energies of the new conducting and 2H-phase branches differ by one order of magnitude at the same gate voltage for both the four-layer and monolayer cases, indicating that the electrical band at the Fermi level was modified. The hysteresis measurements for the gate voltage were performed with a five-layer device, which confirms the reversible electrical transition behavior. The possible origins of the nucleated conducting phase in the depletion region of the 2H phase of MoWSe are discussed.