A hybrid analysis of a continuous-wave terahertz photomixer source structure with plasmonic nano-grating electrodes is presented. Using the hybrid analysis, the enhancement of the optical power absorption due to the presence of the one-dimensional metallic nano-grating is investigated by defining an absorption enhancement factor. We show that the proposed absorption enhancement factor can be used as a design tool, whose maximization provides the optimum geometrical parameters of the nano-grating. Based on drift-diffusion model, the photocurrent enhancement due to the nano-grating electrodes is studied under three different bias configurations. Moreover, the dependence of the photocurrent on the physical parameters of the photomixer is analyzed.