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The onset voltage of a colloid thruster is an important parameter in the design of micromachined thrusters. Because dimensions in MEMS thrusters are in the range of several tens of micrometers and because extractor electrodes can be only 5 μm from the capillary emitters, the aspect ratios for MEMS thrusters are much lower than the one of individual stainless steel or silica glass needles. Therefore the results obtained with established analytical models, that do not consider any parasitic structures, deviate significantly from experimental results on MEMS thrusters. This paper presents a new method to determine the onset voltage of micromachined colloid thrusters by combining electrostatic FEM analysis, including a model of the formation of the Taylor cone, with the equilibrium equation between electrostatic and surface tension stress. The correlation between onset voltage results obtained using the method presented in this paper and experiments are in very good agreement and validate the method for this type of microfabricated thruster.