Metal films grown on an Si wafer perforated with a periodic array of subwavelength holes have been fabricated and anomalous enhanced transmission in the midinfrared regime has been observed. High order transmission peaks up to Si(2,2) are clearly revealed due to the large dielectric constant contrast of the dielectrics at the opposite interfaces. The Si(1,1) peak splits at oblique incidence both in TE and TM polarization, which confirms that anomalous enhanced transmission is a surface-plasmon-polariton (SPP) assisted diffraction phenomenon. Theoretical transmission spectra agree excellently with the experimental results and confirm the role of SPP diffraction by the lattice.