Nanostructures supporting surface plasmon resonance are crucial for efficiency enhancement in thin film solar cells. A plasmonic nanostructure over GaAs layer consisting of Al nanospheres on the front surface and Ag nanocylinders on the rear surface of the GaAs layer with SiO₂ as the back contact is studied for the first time. Using the finite-difference time-domain method, it is found that the optimal nanostructure enhances the short circuit current density by 45.38% and 14.76% in comparison to bare and Si₃N₄-coated thin film GaAs solar cells, while the enhancement in efficiency (η) is 46.83% and 15.11%, respectively. The plasmonic scattering by nanostructure and propagation of waveguide modes at the metal/dielectric interface are mainly responsible for these enhancements.