The properties of the micro-square-prism array are studied theoretically. Its retro-reflective properties are demonstrated. By using the method of geometrical optics, the limit of the total reflection to the incidence and the influence of the available reflective area on the retro-reflectivity are studied. The effect of surface reflection of the array on retro-reflectivity is presented. For contrast, the properties of the micro-triangle-prism array is given as well. The result obtained is a theoretical foundation for designing and manufacturing the micro-square-prism retro-reflector.