A 2.times.2 mechanical optical switch includes a base, four collimators (two input collimators and two output collimators), an ultra thin mirror (double-sided reflection) and a switching mechanism of the mirror. This switch utilizes four collimators, which are arranged 90.degree. with each other on a common plane, to emit and receive parallel light beams at a certain distance. The light beams are reflected by an ultra thin mirror. When the mirror is moved away from the intersection point of two optical paths, the input lights will directly enter into output channels (transmission state). While the mirror is moved to the intersection point which is the reflection position, the optical beams will be bent into alternate output channels (reflection state) simultaneously from both sides of the ultra thin mirror.