Metallic nanostructures provide new possibilities for the photoelectric conversion enhancement of solar cells due to the existence of localized surface plasmon. Since the carrier generation rate is proportional to the electric intensity, a resonant plasmonic excitation can induce a strong local field around metallic nanostructures, hence increasing the optical absorption of the surrounding semiconductor. Previously, the incorporation of metallic nanostructures on the rear side of thin film solar cells has been demonstrated with photocurrent spectrum mapped to the photonic band structure. However, the device structure and the far-field measurement techniques cannot distinguish the photovoltaic enhancement from either or both the increased diffraction or localized surface plasmon resonance. In this work, we perform a three-dimensional finite element simulation to monitor enhanced optical absorption induced by localized surface plasmon and optical diffraction of periodic gold nanospheres incorporated onto the front surface of a silicon solar cell. The calculated structures include gold nanospheres with various diameters and densities. First, the resonance peak of localized surface plasmon is tailored to match the absorption spectrum of silicon. Next, the electric field distribution is calculated explicitly, where we observe enhanced optical absorption by more than 2 fold within a local volume underneath a gold nanpsphere with a diameter of 100 nm, compared to the reference bare silicon. The enhancement is induced by the localized surface plasmon. However, the total absorption of the bulk silicon with gold particles is reduced at the resonance due to a phase cancellation resulting from the transmitted field and the diffracted field. It is therefore important to design the dimensions of nanospheres to achieve broadband absorption enhancement. Finally, the density of nanospheres is also optimized and achieves a maximal photocurrent conversion of 18.6 mA/cm2 at 3.2×1019 cm-2, which represents a compromise between forward scattering and metallic shadowing.