A micro-electro mechanical apparatus having a PN-junction is provided. The micro-electro mechanical apparatus includes a movable mass, a conductive layer, and an electrode. The movable mass includes a P-type semiconductor layer and an N-type semiconductor layer. The PN-junction is formed between the P-type semiconductor layer and the N-type semiconductor layer. The micro-electro mechanical apparatus is capable of eliminating abnormal voltage signal when an alternating current passes through the conductive layer. The micro-electro mechanical apparatus is adapted to measure acceleration and magnetic field. The micro-electro mechanical apparatus can be other types of micro-electro mechanical apparatus such as micro-electro mechanical scanning micro-mirror.