Two experiments were carried out to evaluate the effects of adding phytase on nutrient availability and serum Ca and P level and to determine the effects of phytase on laying performance and egg quality in laying hens. In Exp. 1, twenty four laying hens(1.9kg average body weight and 78.4% egg production) were allotted to four treatments. Treatments included 1) corn-soybean meal based-control diet and 2), 3) and 4) control diet with phytase 200, 400 and 600 unit/kg, respectively. There were no significant effects of treatments on dry matter and nitrogen digestibility(P〉0.05). Ash, Ca and P digestibility in layer fed diet with phytase were greater than those in layer fed control diet(P〈0.05). Laying hens fed diets with phytase 200 and 400 unit retained more Ca than those fed other treatments (P〈0.05). No statistical difference was found for Ca exsretion(P〉0.05). P retention was greater for laying hens fed diet phytase 600 unit than other treaments(P〈0.05). P level in serum was higher for laying hens fed diets with phytase 400 and 600 unit than for laying hens fed other treatments. In Exp. 2, three hundred, IAS Brown layer, 40-week-old, divided into two treatment groups(control vs phytase supplementation without inorganic phosphate in the diets) with five replications per treatment and 30 layers per replication were fed the diets for 6 weeks. Egg production, egg weight and eggshell breaking strength and thickness were not different significantly(P〉0.05). In conclusion, phytase supplementation can be used to increase P utilization and retention in laying hens. Also, phytase supplementation was effective to spare inorganic phosphate in laying hen diets without any adverse effects on production performances.