Optical switches made with micro-electro-mechanical systems (MEMS) technology bear advantages of both MEMS and traditional optics. In this paper, 4×4 MEMS optical matrix switches are proposed, in which on-off micro-mirrors are arranged with crossbar switching network. Ball-lensed single-mode optical fiber collimators are used as input and output ports. With theory of Gaussian beam coupling, the insertion loss (IL) of the optical matrix switches, and also its main origins, i.e. the position misalignments and the angular misalignments, are calculated and analyzed. Some useful conclusions are drawn to guide designing such devices. With the misalignment tolerances: 2 μm of input/output lateral positioning misalignment, 0.15° for input/output angular misalignment, and 0.15° for micro-mirror angular misalignment, the insertion loss of different channels in the pilot study is tested and the highest value of that is 2.77 dB.