Melon is a potential horticultural crop which the production is increased significantly each year in Indonesia. However, a melon plant that has been widely developed in Indonesia faced a problem due to the high tropical temperature that affected the fall of flower causing failure in fruit production. In addition, the variation of sex expression in melon flower is important for genetic analysis and breeding programs. Ethepon is one of plant chemical growth regulator that is known to change the sex expression of the plant by increasing the number of female flower in monoecious plant especially Cucurbitaceae. This research aimed to analyze the effect of ethepon treatment on the sex determination of melon flower. This study was conducted by 3 stages treatment of ethepon during March -July 2014, namely when 2, 5, and 7 weeks after planting. Split-plot design has been used with the main plots were arranged in a completely randomized design by cultivating different melon cultivars i.e: Melodi Gama 1, Melodi Gama 3, Bartek, and PI 371795. The data were analyzed by F-test and Duncan Multiple Range Test (DMRT) with significance level at 5 % using software SAS 9.3. The result showed that ethepon treatment affected the formation of melon flower by increasing the number of female or hermaphrodite flower and decreasing the number of male flowers, especially in the concentration of 75 ppm and 100 ppm but with different responses by different cultivar.