We report an investigation of the impact of surface passivation on the optical properties of ZnO nanorods. Al2O3 coating and hydrogen plasma treatment were used to passivate the surface states. It was found that Al2O3 coating led to the suppression of the deep level emissions, while hydrogen plasma treatment completely quenched the deep level emissions. It was confirmed that the surface states of the as-grown ZnO nanorod arrays indeed contributed to the deep level emissions. Evidence was also provided that shows surface states have a greater impact on the green emission than the orange emission and may cause the negative thermal quenching behavior. Moreover, the passivation effect was confirmed by the changes of the O 1s and Zn 2p spectra.