Abstract: In order to study the early growth and development of valley-type Tibetan sheep, the early development of weight and body size indexes of 50 male and 50 female lambs aged 0 to 18 months were fitted and analyzed by Logistic, Gompertz and Von Bertalanffy models under grazing conditions. The results showed that the development of weight and body size indexes of male and female lambs in valley-type Tibetan sheep had stage characteristics, that is, the growth rate of weight, body length, body height and tube circumference of male lambs was faster than that of female lambs. There was a significant positive correlation between body weight and body size of male lambs at 0-3, 7 and 18 months of age（P＜0.01）, and the correlation coefficient between chest circumference and body weight of male lambs at 0 to 3 months of age was the largest. There was no significant correlation between tube circumference and body weight of 0 month-old lambs（P＞0.05）, but there was a significant positive correlation between body length and body weight（P＜0.05）. In addition, there were extremely significant positive correlations between body weight and body size of other month-old lambs（P＜0.01）. The body weight and body size of valley-type Tibetan sheep at 0 to 18 months of age can be well fitted by Gompertz, Logistic and Von Bertalanffy models. From the fitting degree, Gompertz model had the best fitting effect on body weight. Von Bertalanffy model had the best fitting effect on body size. The limit weight of male lambs was 28.90 kg; inflection point weight was 10.63 kg; age of inflection was 2.99 months, and the maximum monthly weight gain was 3.31 kg. The limit weight of female lambs was 26.91 kg; weight of inflexion was 9.90 kg; age of inflexion was 3.36 months, and the maximum monthly weight gain was 2.44 kg. The results indicated that 0 to 3 months of age was the key period for the early growth and development of valley-type Tibetan sheep, and the nutritional requirements of mother sheep and lambs should be strengthened in this period to give full play to the maximum growth advantages of lambs and shorten the growth cycle.