This paper presents a multi-objective optimization procedure for the design of a sigma micromixer. The procedure combines three-dimensional analyses of fluid flow and mixing, polynomial approximation of objective functions, and a multi-objective genetic algorithm (MOGA). MATLAB Optimization Toolbox (version 7.7, The Mathworks, Inc., MA, USA) was used to invoke MOGA for optimization. A brief discussion on the application of Toolbox is introduced in the paper. Three geometric design parameters concerning the shape of the sidewalls were exploited for optimization. Mixing index and non-dimensional pressure loss were selected as objective functions. For mixing analysis, steady Navier-Stokes equations with a convection-diffusion model for scalar transport were solved at the Reynolds number Re=0.91. The design space was explored through parametric study, and the Latin hypercube sampling method was used as a design-of-experiment technique for selection of the design points in the design space. Surrogate modeling was performed for the objective functions using response surface approximation. Pareto-optimal solutions in the functional space lying on the Pareto-optimal curve were obtained.