Objective To explore the correlation of obesity and early puberty in immature female SD rats by comparison of puberty development-related indicators between the model group and control group after setting up SD immature rat models of nutritional obesity. Methods Ten female rats and 6 male rats were divided in two groups: the parent model group (5 females and 3males) and parent control group (5 females and 3 males). The model group was fed with high-fat diet for 8 weeks and control group was fed with normal diet. The rats of model group with body weight ≥20% higher than those of the control group were matched and their female offsprings were taken as model offspring group. The female offsprings of the parent control group were taken as control offspring group. The 21-days old immature rat model group was fed with high-fat diet for 5 weeks while the control group immature rats were fed with normal diet for the same time. Since 21-days old the immature rats were observed for the vaginal opening state daily, and after the vaginal opening they were examined by vaginal smear. The rats were weighed daily, and their abdominal circumference and body length were measured biweekly. The rats of both experimental groups were killed at the age of 56-days old, the serum triglyceride, cholesterol, estradiol, follicle stimulating hormone, and luteinizing hormone were measured, and the uterus, ovaries, pituitary gland were taken and weighed and the ovary index and uterus index were calculated. The uterus and ovaries were examined by pathology using HE staining, and the corpus luteum and mature follicles in the largest cross section of ovaries were counted. Results 1. The body weight and the abdominal circumference of the 56-days old immature rats in the model group were significantly higher than those of control group (P<0.001). 2. The values of serum TG and CH in the immature rats of model group were significantly higher than those in the control group (P<0.001). 3. The values of serum LH and E₂ in the immature rats of model group were significantly higher, and the value of serum FSH was significantly lower than those of the control group (P<0.001 for all). 4. The number of corpus luteum in the largest cross section of ovaries was significantly higher, and the number of mature follicles was significantly lower in the model group than those of control group (P<0.001 for both). 5. The Pearson correlation analysis showed that the body weight of immature rats was positively related to the serum LH level and quantity of corpus luteum, and negatively correlated with vaginal opening time and the first diestrus time; the abdominal perimeter of the immature rats was positively related to the quantity of corpus luteum, serum LH, and negatively correlated with vaginal opening time and the first diestrus time. Conclusions The early puberty of immature SD rats is positively related to obesity, i.e. the development of immature rats with obesity occurs earlier than in normal immature rats.