【Backgroud】In order to design monitoring methods and possible control strategies, reliable methods are needed for assessing sexual maturity in fieldcaught Bactrocera species. The aim of this study was to provide such a tool for females of the melon fruit fly, Bactrocera cucurbitae (Coquillett), an economically important pest in most of the tropical and sub-tropical areas of the world. 【Method】Ovary length and width, the ovarian index (length width), and egg load of laboratoryreared B.cucurbitae females were recorded daily from eclosion up to 80 days of age. 【Result】The ovarian development could be classified into four stages and six grades according to ovarian morphological characteristics: previtellogenesis (stages Ⅰ and Ⅱ), vitellogenesis (stages Ⅲ and Ⅳ), gravid (stage Ⅴ), parous (stage Ⅵ). As the ovaries were developing, their length and width increased rapidly between stage Ⅰ and stage Ⅴ, but tended to decrease at stage Ⅵ because of ovulation. The ovaries had approximately equal length and width during the previtellogenesis (stages Ⅰ and Ⅱ), after which ovaries were longer than wide. Egg chambers became visible during the vitellogenesis phase, during which the volume of oocytes increased rapidly and yolk started to accumulate. Mature oocytes became visible during stage Ⅴ, when the whole ovaries were cylindrical in shape. As ovulation was initiated, the egg load of parous females was significantly lower than that of gravid females. Despite the ovary length at stage Ⅳ being greater than at stage Ⅵ, both ovary width and the ovarian index were similar in these two stages. As the yellow follicular relicas could be seen in stage Ⅵ, stage Ⅳ and stage Ⅵ could be distinguished by ovarian length combined with follicular relicas. 【Conclusion and significance】The potential applications of this technique to evaluate the physiological age structure of B.cucurbitae females captured in color sticky traps are discussed.