An improved breakdown voltage(BV) SOI power MOSFET with a reduced cell pitch is proposed and fabricated. Its breakdown characteristics are investigated numerically and experimentally. The MOSFET features dual trenches(DTMOS),an oxide trench between the source and drain regions,and a trench gate extended to the buried oxide(BOX). The proposed device has three merits. First,the oxide trench increases the electric field strength in the x-direction due to the lower permittivity of oxide( εox) than that of Si(εSi). Furthermore,the trench gate,the oxide trench,and the BOX cause multi-directional depletion,improving the electric field distribution and enhancing the RESURF(reduced surface field) effect. Both increase the BV. Second,the oxide trench folds the drift region along the y-direction and thus reduces the cell pitch. Third,the trench gate not only reduces the on-resistance,but also acts as a field plate to improve the BV. Additionally,the trench gate achieves the isolation between high-voltage devices and the low voltage CMOS devices in a high-voltage integrated circuit(HVIC),effectively saving the chip area and simplifying the isolation process. An 180 V prototype DTMOS with its applied drive IC is fabricated to verify the mechanism. An improved breakdown voltage (BV) SOI power MOSFET with a reduced cell pitch is proposed and fabricated. Its breakdown characteristics are investigated numerically and experimentally. The MOSFET features dual trenches (DTMOS), an oxide trench between the source and drain regions, and a The proposed device has three merits. The proposed device has three merits. The proposed device has three merits. First, the oxide device increases three of the electric field strength in the x-direction due to the lower permittivity of oxide (εox) than that of Si (εSi) . The trench gate, the oxide trench, and the BOX cause multi-directional depletion, improving the electric field distribution and enhancing the RESURF (reduced surface field) effect. Both increase the BV. Second, the oxide trench folds the drift region三, the trench gate not only reduces the on-resistance, but also acts as a field plate to improve the BV. gh-voltage devices and the low voltage CMOS devices in a high-voltage integrated circuit (HVIC), effectively saving the chip area and simplifying the isolation process. An 180 V prototype DTMOS with its applied drive IC is fabricated to verify the mechanism.