Abstract: For planar periodic structure with finite thickness, the paper presents an algorithm, integral equations solved by method of moments (IE-MoM), making a quickly and effectively calculation of the electromagnetic characteristics. A detailed electromagnetic characteristics analysis is done about frequency selective surfaces (FSS) with four-legged loaded elements. The paper presents the reflection and transmission coefficients, radiation pattern, and surface electric field distribution for the FSS with different thickness. Through the analysis, it is found that the thickness of the metal screen makes a difference for the electromagnetic characteristics of the FSS in higher frequency resonant. In addition, a good agreement can be observed between the results of simulations obtained from our approach and with the commercial software HFSS. The computer time is also improved greatly.