Present study was conducted to investigate effects of microbial phytase in laying hen diets on utilization of non-phytate phosphorus(NPP) whose levels were adjusted to be adequate or lower than that of NRC requirements. Birds of control roup were fed a diet containing 0.275% NPP and 3.4% Ca, satisfying the NRC(1994) feeding standard. bird on T1, T2 and T3 were allowed to eat diets containing NPP at 100, 80 and 60%, respectively, of Control group, and 4.0% Ca level along with a microbial phytase added at a level of 300 DPU. Three hundred and sixty, ISA Brown layers, 23-week-old, divided into four treatment groups with three replications per treatment and 30 layers per replication were fed the diets for 12 weeks. Levels of feed intake were not different among the groups, The egg mass/feed intake ratio appeared better in T2 group by about 8%, though without a statistical significance, compared to that of control. Egg production fate tended to be improved over the control group by feeding the 100%(T1) and 80%(T2) NPP diets added with phytase, with a significant difference for T2(p<0.05). Mean egg weight and egg shell quality, measured by breaking strength and thickness of the egg shell, of the T2 group tended to show numerically better, without a significance than those of control. Furthermore, birds of the T2 group showed higher calcium and phosphorus contents in tibia by about 9%(p<0.05) than the control. Overall performances of birds in T1 appeared better than those of control, but tended to be lower than those of the T2. The birds in T3 performed similar to the those of the other dietary groups except the relatively low tibia calcium level. In conclusion, the results of this study suggest that supplementation of microbial phytase at a level of 300 DPU was effective to spare about 20% of NPP in laying hen diets without any adverse effects on production performances and bone quality.