Longitudinal transmission-line modal theory is applied to analyze maximum power transfer in plasmonic grating-assisted directional couplers (P-GADC) based on silicon waveguide. By defining a coupling efficiency amenable to rigorous analytical solutions and interference between even and odd modes, the power exchange of TE modes as a function of propagation distance is evaluated. The numerical result reveals that maximum power transfer occurs at a grating period ${Lambda}_{eq}=10.26{mu}m$, in which the insertion loss of supermodes is equal to each other. That is, it is generally different from conventional phase-matching condition or minium gap condition of GADC.