Abstract: The objective of this study was to assess the effects of commercial β-mannanase enzyme supplementation in low-energy corn-soybean meal diets on performance, intestinal morphology and mRNA expression of intestinal tight junction proteins in 1-21 days broiler chickens. The 1800, 1 day old broiler chicks were divided into 5 treatment groups, 6 pens in each treatment. The study was performed in a randomised complete block design. The 5 treatments were Positive diet Group (PG) with basal energy level, Negative diet Group (NG) with lower energy of 502.08 kJ kg-1 and other groups supplemented with 150, 300 and 450 mg kg-1 β-mannanase based on NG. Decrease of energy in NG resulted in loss of the Average Daily Gain (ADG) (p<0.05) and Feed Conversion Ratio (FCR) (p<0.05). Based on the NG, supplementation with β-mannanase in diet significantly improved the Average Daily Gain (ADG) (p<0.05) and Feed Conversion Ratio (FCR) (p<0.05) of birds. ADG of groups with enzyme and lower energy reached to the level of PG. Supplementation with 300 g ton-1 β-mannanase increased the villus height of the small intestine (p<0.05) and 450 g ton-1 β-mannanase significantly decreased the crypt depth (p<0.01). The addition of β-mannanase at 150, 300 and 450 g ton-1 to the diets increased the ratio of crypt depth to villus height of the duodenum and jejunum in birds (p<0.01). The ratio of crypt depth to villus height of the ileum was also increased with 300 and 450 g ton-1 β-mannanase supplementation compared to the low-energy diet group (p<0.01). Chicks fed diet with the β-mannanase showed an increased mRNA expression of ZO-1 in the duodenum compared with the low energy diet (p<0.05). Supplementation with 300 and 450 g ton-1 β-mannanase to the diets enhanced the mRNA expression of Occludin and ZO-1 in the jejunum (p<0.05) and 300 g ton-1 β-mannanase increased enhanced the mRNA expression of Occludin and ZO-1 in the ileum (p<0.05). Thus, the addition of β-mannanase to low energy diets improved the performance, gut morphology and mRNA expression of intestinal tight junction proteins.