Positioning of forward symmetrically installed square light-emitting diode (LED) arrays are studied to achieve uniform illumination with which high quality images can be gathered for machine vision measurement. The optimization method is used to solve this problem because the optical axes of the arrays are not perpendicular to the irradiated plane, which can′t be solved by analytic method. The variance is chosen to establish the mathematical model of goal function on the base of the structure characteristics of LEDs. The simulated annealing (SA) algorithm is chosen for solving the non-convex objective function. An experiment system is developed for illumination photometry. The sizes of area with 95% of uniformity of the analytic method are a little larger than those of the optimization based method. The maximum difference is within 4%. The results show that the optimization objective is reasonable and the method is feasible.