• Nanoemulsions of some natural oils i.e., clove, black seed, orange and lemon and their fungicidal activity against postharvest fruit rot of cucumber were prepared and studied. • Nanoformulations of clove and black seed essential oils (2:1) reduced mycelial linear growth, andcompletely inhibited (100%) conidia sporulation of fungal isolates. • Nanoformulation of clove and black seed essential oils (2:1) respectively is a promising safe alternative fungicide for application against postharvest rot of cucumber fruits management. There are growing concerns about the extensive use of synthetic fungicides worldwide due to their negative health effect on humans, ecosystem, and the development of fungal resistance. Therefore, developing new and safe eco-friendly antifungal agents such as plant-based oils is of major interest. This study was conducted for the first time to develop and evaluate the efficiency of eco-friendly nanoemulsions of some natural oils and evaluating of its efficiency against postharvest fruit rot of cucumber caused by the following fungal isolates: Galactomyces candidum (MF373433), Alternaria tenuissima (MF 373440) and Fusarium solani (MF373443). Nanoemulsions of clove, black seed, lemon, and orange oils were formulated by using ultrasound and subjected to physicochemical and stability studies. The efficiency of these nanoemulsions was evaluated against the postharvest fruit rot fungi of cucumber. The oils of promising nanoemulsions have been analyzed by gas chromatography-mass spectrometry analysis (GC/MS). The acute toxicity study was conducted on rats and on the luminescence emitted of Vibrio fischeri bacteriausing Microtox®. The smaller droplet size of nanoemulsions was found after 10 min of sonication with an oil to Tween 80 ratio of 1:1 and 1:2 for clove and black seed oils, respectively. Nanoemulsion of black seed and clove oils from Egypt at 5000 ppm showed a significantly high reduction in the linear mycelial growth of fungal isolates. The mixture formula (2:1 v/v) of nanoemulsions of clove and black seed oils showed a significantly high reduction in linear mycelial growth, completely inhibited (100%) conidia sporulation of fungal isolates, and highly reduced the percentage of the rot of germinated seeds as well mortality of cucumber developed seedlings. Nanoemulsion formulation of clove and black seed (2:1) at concentration 2.0 % as soaking treatment of cucumber fruits before artificial infestation by inocula of fungal pathogens caused completely (100%) suppress postharvest fruit rot of cucumber. Both clove and black seed oils nanoemulsions and their combination showed no signs of toxicity or mortality in rats and no toxic effects on the luminescence emitted of Vibrio fischeri bacteria using Microtox® assay. Therefore, it can be concluded that the nanoformulation of clove and black seed oils at the ratio of 2:1 is a promising green and safe alternative to synthetic fungicide for application against postharvest rot of cucumber fruits. [ABSTRACT FROM AUTHOR]