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This paper reports an improved method of fabricating an ultra deep (40-120 μm) and high aspect ratio (more than 25: 1) electrical isolation trench without void to increase the mechanical strength and reliability of the isolation trench in monolithic integration of bulk micromachining MEMS sensors. Before etching and refilling the trench, 0.1 μm oxide and 3-4 μm polysilicon are LPCVD deposited on the wafer surface as sacrificial films. After etching the trench by DRIE, such sacrificial films are removed to enlarge the trench opening. Then this trench is refilled well without voids using LPCVD oxide and polysilicon. Such an electrical isolation trench has been used in the monolithic integration of a bulk-micromachining MEMS gyroscope, which has shown high performance. Such an isolation trench has sufficient mechanical strength to sustain the bulk MEMS structure. Electrical test shows that such an isolation trench can electrically isolate the MEMS structures effectively from one another and from the on-chip detection electronics. The average resistance in the range of 0-100V is more than 1011 Ω and no breakdown under 100V.