We investigated the influences of the defects generated by plasma on a silicon (Si) microcantilever. The E' center density of the microcantilever was drastically increased after Ar plasma irradiation. The mechanical characteristics, including resonant frequency (f) and the mechanical quality (Q) factor of a microcantilever were drastically decreased by plasma irradiation, which revealed that plasma irradiation deteriorated the mechanical characteristics of the micro element. Our results revealed that deterioration of f and Q factor depended on Young's modulus of microcantilevers and the ratio of defect depth to microcantilevers thickness respectively. Furthermore, These results showed considerable impact on Micro- and Nano-Electro-Mechanical Systems.