A fabricating method of a microelectronic device including the following steps is provided. First, a substrate is provided. Second, a semi-conductor element is formed in a CMOS circuit region of the substrate. Next, a plurality of metallic layer, a plurality of contact plugs and a plurality of oxide layer are formed on the substrate. The metallic layers and the oxide layers are interlaced with each other and the contact plugs are formed in the oxide layers and connected with the metallic layers correspondingly so as to form a micro electromechanical system (MEMS) structure within a MEMS region and an interconnecting structure within the CMOS circuit region. Then, a first protective layer is formed on at least one of the oxide layers and a second protective layer is formed on the interconnecting structure. Predetermined portions of the oxide layers located within the MEMS region are removed and thereby the MEMS structure is partially suspended above the substrate. The present invention also provides a microelectronic device, a MEMS package structure and a fabricating method thereof.