Ninety Isa Brown layers from 51 to 61 weeks of age were allocated to three treatment groups, namely H0, H1 and H2. Control (H0) hens were fed a commercial diet while H1 and H2 hens were fed a diet including 30 and 90 ppm dietary humic acid in liquid form, respectively. Hens were placed individually in cages in three tier batteries. Egg production (% hen-day) in the H2 group was higher (P < 0.05) than control group. Egg mass of H2 hens was higher (P < 0.05) than H0 and H1 hens. Total feed intake in H2 group was significantly higher (P < 0.05) than H1 group. Egg shell thickness of H1 hens was higher (P <0.05) than H2 hens. Egg shell strength of H1 hens was higher (P < 0.05) than control and H2 hens. Egg weight, feed conversation ratio and yolk weight were not affected by dietary humic substances. These results indicated that the supplementation of 30 ppm humic acid into the diet may increase the egg shell strength without affecting egg production and feed efficiency compared to control counterparts.