A single-mask dry-release process for fabrication of high aspect ratio SOI MEMS devices is presented,which takes advantage of the lag effect in silicon DRIE(deep reactive ion etching).The wide trenches and the releasing holes are etched to the buried oxide in the first-step DRIE whereas the narrow trenches are still connected due to the lag effect.After the buried oxide is removed by wet etching through the opened releasing holes and wide trenches,the narrow trenches are etched through by the second-step DRIE.Not only can the sticking problems be avoided,but also the footing effect during the DRIE can be partially suppressed.The feasibility of the proposed technique was verified by implementing a capacitive accelerometer.The scale factor and the non-linearity of the fabricated accelerometer were measured to be 63.4 mV/g and 0.1% with the measurement range of ±1 g,respectively. A single-mask dry-release process for fabrication of high aspect ratio SOI MEMS devices is presented, which takes advantage of the lag effect in silicon DRIE (deep reactive ion etching). The wide trenches and the releasing holes are etched to the buried oxide in the first-step DRIE while the narrow trenches are still connected due to the lag effect. After the buried oxide is removed by wet etching through the opened releasing holes and wide trenches, the narrow trenches are etched through by the second-step DRIE. Not only can the sticking problems be avoided, but also the footing effect during the DRIE can be partially suppressed. The feasibility of the proposed technique was verified by implementing a capacitive accelerometer. The scale factor and the non-linearity of the fabricated accelerometer were measured. to be 63.4 mV / g and 0.1% with the measurement range of ± 1 g, respectively.