Abstract: 【Objective】 The study of secretory cavity development and essential oil in the fruit of Citrus reticulata may provide scientific basis of morphology and phytochemistry for timely harvest of Chenpi. 【Method】The developmental processes of fruit and secretory cavity were studied by paraffin section and semi-thin section method. The change of essential oil during fruit development was quantitatively analyzed. 【Result and conclusion】 From May to January, the fruit of C. reticulata developed from hard and dark green to soft and golden yellow or orange. In the epicarp, the secretory cavities were distributed in a ring with different depths. The developmental processes of secretory cavity were divided into four stages. The secretory cavity was shown to be composed of two areas: a cap area and a circular area. In the center of the latter, one big cavity was surrounded by 2-5 layers of epithelial cells. Essential oil secreting from the epithelial cells was stored in this cavity. With the fruit development, the accumulation of essential oil followed a “S” trend. The ratio of essential oil displayed a gentle rise during the period from May to August, a rapid growth from September to November, reaching a peak in the middle of November, and a slight decline in early December. It is suggested that the medically effective fraction of Chenpi is the epicarp. The best time for harvest should be from late Novermber to early December. In this period, the fruit color is light yellow or golden yellow. The average diameter of secretory cavities reaches the maximum (200 μm), and the ratio of essential oils is also the highest (8.2%-8.5%).