Monitoring of changes in nutrients composition of vegetables during different stages of maturity help to determine optimal harvesting date; therefore, collecting vegetables with higher nutritional value and eating quality. Changes in chemical composition as well as total phenolic and flavonoids content of three Zucchini cultivars; namely, Eskandrany (ES), Amjad F₁ (AMF₁), and New Eskandrany F₁ (NESF₁) grown in Egypt, with development of maturity stage were investigated. The results showed that dry matter, protein, oil, ash, and crude fiber contents of Zucchini vegetables were decreased with the advancement of maturity stage. However, gradual increase in total carbohydrate content was observed with the development of maturity. In addition, total phenolics and flavonoids contents were decreased in the more developed fruits. Based on the obtained results, it could be concluded that although there is an increase in yield of vegetables at later maturity stage, but vegetables at early stage of maturity contain high content of nutrients and health promoting compounds, such as protein, ash, crude fiber, and phenolics and flavonoids.