Abstract: Influence of slot width on transmission properties of plannar FSS(frequency selective surfaces) was explained by physical essence. With the width of element slot in FSS increasing, it could shorten resonant wavelength of the element. Correspondingly it could increase the resonant frequency of the element. Influence of slot width on transmission properties of plannar FSS was calculated with Floquet theorem, and the small rectangle element was machined successfully by etching technique. Effects of the slot width on the transmission properties of FSS waspreliminarily explored by experiment. The experimental results show that the resonant frequency of FSS increases with the slot width, and the transmission ratio and resonant bandwidth is becoming greater. The experimental data accord with the computed results within error permissibility.