The surface electromagnetic field enhancement in a sub-wavelength metallic groove array is studied with rigorous coupled wave analysis for surface enhanced Raman scattering. The effects of groove depth and array period on electromagnetic field enhancement at the rabbet of a groove array are simulated. The contribution of resonance effect in structure to electromagnetic field enhancement is achieved. The average enhanced effect of Raman scattering of sub-wavelength metallic groove array and the effective range of electromagnetic field enhancement out of the array are investigated. The results show that with the contribution of resonance in grooves, the Raman scattering enhancement factor of sub-wavelength metallic groove array in one period can reach 106 orders of magnitude. Obvious electromagnetic field enhancement can be obtained at vertical distance to the array surface within 1/5 wavelength, out of which enhancement factor declines rapidly to unitary magnitude. The numerical results show that the surface electromagnetic field enhancement of the sub-wavelength metallic groove array depends weakly on the type of noble metal.