A total of 288 Ross 308 broiler chickens (mixed sex) were used to investigate the effect of supplemental guanidino acetic acid (GAA) to soybean meal (SBM) and canola meal (CM) based diets on growth per-formance, carcass characteristics, liver enzymes, and morphometric indexes. Birds were randomly assigned to four treatments in a completely randomized design with a 2 × 2 factorial arrangement consisting of 2 types of oil seed meal (SBM and CM) and 2 levels of GAA (0 and 0.6 g/kg) with six replicates in each. The results indicated that birds fed with CM diet had significantly lower feed intake and body weight gain and a worse feed conversion ratio than those fed with SBM diet (P<0.05). Supplementing GAA to the CM diet but not to the SBM diet led to increased feed intake so that the value was similar to those birds receiving the SBM diet as a statistical point (P<0.05), while the improvement of body weight gain was not achieved completely. Carcass, breast, and thigh yields were statistically lower in birds fed the CM diet than those fed the SBM diet, while the abdominal fat and liver weight were higher (P<0.05). Neither type of oil seed meal nor GAA significantly affected the liver enzymes and nitric oxide concentrations, as well as gut mor-phometric indexes (P<0.05). It can be concluded that supplementing 0.6 g/kg GAA to the CM-based diet to some extent improves the growth performance of broiler chickens, and more content may be needed for complete improvement. Manuscript profile