Early development of small intestine can cause higher growth rate in hatchlings. Therefore, the aim of this study was to evaluate the effect of in ovo injection of different ratios of DL-methionine to L-lysine on small intestinal histomorphology and immune system organs of day-old Ross 308 broiler chicks. In this experiment, 210 fertile Ross 308 broiler breeder eggs were randomly assigned into 7 experimental groups including 30 individual eggs per group. Treatment groups included: sham control (non-injected), control (1ml distilled water), 40. 5, 45. 5, 50. 5, 55. 5, and 60. 5% DL-methionine to L-lysine ratios which were injected into the amnion at 14 d of incubation. On day 22, hatched chicks were weighed and slaughtered in order to evaluate the characteristics of small intestine and immune system organs. Based on the results of the present experiment, the effects of in ovo injection with different DL-methionine to L-lysine ratios on weight and relative weight of ileum, ileum length, weight and relative weight of small intestine, length and relative length of small intestine were significant (p<0. 01) and elevatory. Also, significant effect (p<0. 05) of treatments was observed on crypt diameter (except for duodenum), crypt depth, villus height, and villus height to crypt depth ratio in duodenum, jejunum, and ileum. On the other hand, immune system organs were not affected by the treatments. The overall results of the present study showed that in ovo injection from 40. 5 to 50. 5% of DL-methionine to L-lysine ratio improved small intestinal growth and histomorphology in broiler chicks.