A new plate Static micromixer interconnected with or-type unit elements in series has been developed. which has systematic mixing functions of division and multiplayer formation of fluid layers by combining splitting, realignment and recombination of fluid streams. The number of unit elements required for complete mixing n was measured by conducting the decolorizing reaction of iodine with sodium thiosulfate for various Reynolds numbers Re and flow ratios of two fluids to be mixed. At low Re, n increased with Re and its variation with operating variables and device dimensions were well correlated by use of a functional relation derived from a model analysis for the diffusion process in a fluid layer whose thickness is reduced by half after the fluid passes through each unit element. When Re exceeds 10, n begins to decrease with Re. Under the condition of large Reynolds number, CFD analysis shows that a large deformation and stretch of the fluid interface takes place due to the bending and winding channel structure of or-type element, and the mixing rate is accelerated through the deformed and stretched interface. n was correlated experimentally with Re and a variable representing Schmidt number derived from the model. In addition, the effects of aspect ratio were investigated by CFD analysis. In order to promote ideal fluid mixing of splitting and recombination and efficient removal of heat generated by reaction in sigma-type plate static mixer, it is recommended to adopt channels whose cross-section is as close as possible to a square.