This experiment was conducted to determine the effect of replacing limestone by sea shell on digestibility, egg strength and thickness, Ca and P of serum and Ca and P of tibia in layer hens. A total 54 brown layers was assigned to the treatments. There were six layers per replicate with three replicate per treatment. Treatments were layer diet with 7.5% limestone as control, 7.0% limestone and 0.5% sea shell in diet(SS0.5) and 6.5% limestone and 1.0% sea shell in diet(SS1.0). Digestibility of DM was significantly difference between SS0.5 and SS1.0(P〈0.05). Digestibilities of Ca and P were not affected by treatments(P〉0.05), however, SS1.0 tended to increase Ca and P digestibility compared to control. Egg thickness and egg strength were no differences among the treatments(P〉0.05). Differences of initial and finial period on Ca of plasma were significantly affected in the a.m. between SS0.5 and SS1.0(P〈0.05). However, there was no difference in Ca of plasma in the p.m. among the treatments and Ca of plasma in control had a tendency to decrease in the p.m. compared to that of treatments(P〉0.05). Ca of tibia was significantly increased by SS1.0 compared to control(P〈0.05). However, there was no difference in P of tibia among the treatments(P〉0.05). Based on these results, dietary supplemental sea shell mat have a role to improve the egg quality of layer hens.