The present study was conducted to evaluate the effect of 20 d starvation on growth, survival, histomorphology and ultrastructure changes in the digestive system of juvenile red swamp crayfish (Procambarus clarkii). Juveniles were divided into two groups: a food-deprived group and a control group at 9 day after hatch (DAH). Individuals were sampled at 14, 20, 29 DAH. During the 20 d fasting period, the mean body weight and total body length of crayfish fluctuated around 10.17 mg and 8.12mm respectively, and the mortality was zero. Histomorphological changes of digestive system were observed in the food-deprived group after 20 days of starvation: the esophagus and stomach walls were thinning, the epithelium atrophied to cuboidal, nuclei were darker and smaller, and nucleolus was difficult to observe; the midgut and hindgut showed wider volume, thinning wall, atrophied epithelial and muscularis and shorter ridges; and hepatopancreas tubule lumens were wider, the lipid droplets in R-cells were smaller and less, and the quantity of typical B-cells decreased. Changes in the ultrastructure of starved crayfish were also observed: the mitochondria of midgut epithelium and R-cells were swollen and vacuolated, and the ridges of which were fractured and reduced. In addition, the electron density of cytoplasmic matrix of R-cells decreased, and the quantity of glycogen granules and lipids also decreased. Changes in the ultrastructure of B-cells were similar to those of R-cells. Though degeneration in histological structure and function of digestive organs were obvious during starvation, juvenile P. clarkii was able to endure a relative long-term starvation.