Based on the theoretical formalism derived from the diffraction theory and electromagnetic boundary condition (Phys. Rev. Lett. 167401( 2003)), we investigate numerically the diffraction behavior of light passing through a subwavelength metallic slit-grooves structure, slit surrounded symmetrically by a finite array of grooves at the output surface. The diffraction dependence with the geometrical parameters, groove depth, width and number is analyzed in detail. It is found that variant profile of angle spectra and beaming intensity can be obtained in the far field by tuning the geometrical parameters. Numerical analysis shows that these diffraction behaviors are associated with different excitation states of surface electromagnetic wave. (c) 2006 Optical Society of America.