We have studied the impact of mesa sidewall passivation by SiO2 on characteristics of III-nitride/silicon tandem solar cells. These dual junction solar cells were fabricated from standard n-type Si (111) substrates with III-nitride epitaxial layers grown by plasma-assisted molecular beam epitaxy (PAMBE). Photovoltaic testing was experimentally carried out under a solar simulator before and after the 100nm-thick SiO2 passivation of these solar cell mesa side walls. We have found that the sidewall passivation improves the efficiency of these solar cells. The open-circuit voltage (VOC) increased from 1.45 to 1.53 V, the short-circuit current density (JSC) enhanced from 0.116 to 0.121 mA/cm2, the fill factor increased from 39.7 to 41.5% under the solar simulator illumination yielding 13.3% conversion efficiency improvement after passivation. Moreover, IPCE (the incident monochromatic photon to current conversion efficiency) also moderately increased by approximately 13% in the visible region after passivation.