Abstract The present experiment was designed to investigate effects of supplemental guanidinoacetic acid in standard or low energy diets with mash or pelleted form on growth performance, intestinal morphology and blood attributes of broiler chickens. A total of 360 one-day-old broiler chickens (Ross 308) were assigned to 6 treatments and 5 replicates of 12 chicks each, based on a completely randomized design during the starter (1 to 14), growing (15 to 28), and finisher (29 to 42) periods. Experimental treatments were pelleted or mash diets with standard or 150 kilocalories deficient in energy contents. A 1.2 g/kg guanidinoacetic acid was supplemented to low energy diets (Table 1). All chickens were evaluated for growth performance [body weight (BW), daily weight gain (DWG), daily feed intake (DFI), and feed conversion ratio (FCR)], carcass traits (carcass yield, abdominal fat, heart, liver, pancreas, proventriculus, gizzard, and small intestine), intestinal morphology (villus height, crypt depth, and villus height to crypt depth ratio), and serum biochemical parameters (uric acid, creatinine, total protein, homocysteine). Results indicated that chickens received pelleted diets had improved DWG and (FCR) compared with broilers fed on mash diets (P < 0.05; Table 2). Broilers received mash diets with low energy contents had compromised DWG and FCR which ameliorated after dietary guanidinoacetic acid supplementation equal to standard energy diets (P < 0.05). Furthermore, supplementation of guanidinoacetic acid in pelleted diets deficient in energy content could compensate growth only during the entire rearing period (P < 0.05). Supplemental guanidinoacetic acid decreased liver weight (P < 0.05) and decreased heart weight when supplemented in mash diets (P < 0.05; Table 3). Reduction in dietary energy content increased villus height to crypt depth ratio in all 3 intestinal sections of dedunum, jejunum and ileum (P < 0.05) but supplementation of guanidinoacetic acid did not change the noted intestinal morphology. Reduction in dietary energy content of mash or pelleted diets increased serum acid uric concentration (P < 0.05) while decreased when diet of broilers added with guanidinoacetic acid (P < 0.05). Serum creatinine concentrations increased following dietary guanidinoacetic acid supplementation (P < 0.05; Table 4). Overall, supplementation of 1.2 g/kg guanidinoacetic acid improved growth performance of broiler chickens fed on energy deficient diets with more pronounced effects in mash diets. Moreover, supplemental guanidinoacetic acid affected blood attributes but failed to influence the effect of dietary energy variations on the intestinal morphology of broiler chickens.