Citrus black rot is a serious disease of citrus plants caused by <i>Alternaria citri</i>. The current study aimed to synthesize zinc oxide nanoparticles (ZnO-NPs) by chemically or green method and investigate their antifungal activity against <i>A. citri</i>. The sizes of synthesized as measured by transmission electron microscope of ZnO-NPs were 88 and 65 nm for chemical and green methods, respectively. The studied prepared ZnO-NPs were applied, <i>in vitro</i> and <i>in situ</i>, at different concentrations (500, 1,000, and 2,000 µg/ml) in post-harvest treatment on navel orange fruits to verify the possible control effect against <i>A. citri</i>. Results of <i>in vitro</i> assay demonstrated that, at concentration 2,000 µg/ml, the green ZnO-NPs was able to inhibit about 61% of the fungal growth followed by 52% of chemical ZnO-NPs. In addition, scanning electron microscopy of <i>A. citri</i> treated <i>in vitro</i> with green ZnO-NPs showed swelling and deformation of conidia. Results showed also that, using a chemically and green ZnO-NPs at 2,000 µg/ml <i>in situ</i> in post-harvest treatment of orange, artificially-infected with <i>A. citri</i>, has reduced the disease severity to 6.92% and 9.23%, respectively, compared to 23.84% of positive control (non-treated fruits) after 20 days of storage. The out findings of this study may contribute to the development of a natural, effective, and eco-friendly strategy for eradicating harmful phytopathogenic fungi.