SummARyThis experiment was carried out to evaluate the effects of four levels of dietary threonine (Thr) supplementations on growth performance, gut size and histomorphometric alterations of small intestines in broiler chicks in 1-14 days. Two hundred eighty-eight Ross 308 one-day-old male broiler chicks were randomly assigned to four treatments with six replicates (12 birds per replication) received a common diet based on corn, wheat and soybean meal that met nutrients requirement of Ross 308. Birds were fed dietary treatments consisting of four levels of standardized digestible (SD) Thr: control diets (Basal diets) containing 0.65% SD Thr, 0.89% SD Thr (nutrients requirement of Ross 308), 0.93% and 0.97% SD Thr. Body weight gain (BWG), feed intake (FI) and feed conversion ratio (FCR) were measured at 1, 7 and 14 days old. Morphometric analysis of small intestine was carried out to investigate the effect of Thr levels on development of small intestines in starter period of broiler chicks. Results indicated that Thr levels improved FCR (quadratic effect, p=0.044) and BWG (quadratic effect, p=0.0009) for broilers given the diets containing 0.89% SD Thr in the starter period compared to other treatments (p<0.05). Furthermore, villus height and muscular thickness in duodenumand muscular thickness in jejunum were increased by this treatment (p<0.05). Villus width was increased by Thr treatments comparing to control group, as well. In conclusion, broiler performance and intestinal morphometry were improved by Thr supplementation which were efficiently obtained by 0.89% SD Thr in the first two weeks of the broiler´s diet.