The transmission distribution of transverse electric wave (TE) through one-dimensional metallic gratings with sub-wavelenghth slits with dielectric added in different positions has been simulated by the FDTD (finite difference time domain) method, the extraordinary transmission can be produced at several special wavelengths. The simulation shows that the guided-mode-like resonance theory is the main reason giving rise to the first peak and the second peak. Based on the guided-mode-like resonance theory, from the Maxwell’s equations,we nade sure that the Fabry Perot resonance is the main cause producing the third peak by the effective index mehod. In a word, the guided-mode resonance theory and the Fabry-Perot-like resonance reveal the physical nature of the extraordinary transmission phenomenon which provides a complete theoretical basis for further study of the extraordinary transmission phenomenon of TE-polarized wave.