The electromagnetic enhancement generated by a single subwavelength groove in metallic substrate is analyzed for the application of surface enhanced Raman scattering (SERS) on metallic micronano-structure. The electromagnetic field distribution out of the groove illuminated by a plane-wave is calculated using fully-vectorial numerical method. The rigorous fully-vectorial data are well predicted by a simple semi-analytical Fabry-Perot (F-P) model. The computation amount for designing the optimal groove depth can be saved by using the model. Physical mechanism of electric-field enhancement out of the groove, based on the model is analysed, which shows that the enhancement is mainly attributed to the transmitted field of enhanced fundamental groove mode under resonance. The range of the enhanced electric-field out of the groove is studied. The results show that, for different groove widths and illumination wavelengths, the electric-field enhancement is considerable at vertical distances to the groove mouth less than 0.1 times of the wavelength and that out of this range, the electric-field enhancement factor attenuates rapidly to about unitary magnitude.