A novel orthogonal simulated annealing algorithm OSA for optimizations of electromagnetic problems is proposed in this paper. High performance of OSA mainly arises from an intelligent generation mechanism (IGM) based on orthogonal experimental design (OED). The OED-based IGM can efficiently generate a good candidate solution for next move by using a systematic reasoning method instead of the conventional method of random perturbation. It is shown empirically that OSA performs well in solving parametric optimization problems and in designing optimal electromagnetic devices, compared with some existing optimization methods using simulated annealing algorithms and genetic algorithms.