The current CMOS-based analog-to-digital converter (ADC) is confronted with the performance limits. These circumstances propose the necessity for new approaches for the ADC. This paper suggests a MEMS-based ADC of field emission. On/off state is determined by the existence of electron emission between the tip electrodes pair, which is used to encode the thermometer code. The abrupt increase of the current on the turn-on threshold voltage functions as the border of on/off state. 8 resolutions of 3-bit flash type ADC is made with 7 tip electrodes pairs of different turn-on threshold voltage. Turn-on voltages are differentiated with the different electrodes separation gap. Suggested method has the potential of low power consumption and size reduction.