Black pod disease caused by Phytophthora palmivora is a significant disease on cocoa crops worldwide. Citronella and geraniol are two dominant substances of citronella oil that have fungicidal activity, which is potentially used to control black pod disease. However, those two substances are relatively unstable form volatile. To improve stability can be done using nanotechnology. The study aimed to analyze the effectiveness of citronella-oil-nanoemulsions formulas on the growth of P. palmivora in vitro and on cocoa pods. Eight nano formulas were prepared at the Postharvest Center in Bogor; however, the effectiveness was conducted at the Indonesian Industrial and Beverage Crops Research Institute in Sukabumi. The nano formula was prepared following spontaneous, simultaneous, and inversion methods. The experiments were designed in a completely randomized design, with five replications. Eight treatments were evaluated, i.e., the inversion formulas (F1, F2, F5 & F7), spontaneous (F3, F6 & F8), and simultaneous (F4). Two concentration levels of the formula were tested, i.e., 0.5% for in vitro and 1.0% in cacao pods. The variables observed were the inhibition rate of formula, bioassay, concentration, and inhibition rate on cacao pods. The nanoemulsion formulas of citronella oil could inhibit 23.13%–77.78% of the growth of P. palmivora. Formula F5 was the best, which inhibited 77.78% growth in vitro and 31.60% in cacao pods. The nanoemulsions of citronella oil affected the growth rate, fresh weight, and dry weight of P. palmivora. For better effectiveness of the formula on cocoa pods, the concentration should be increased.