Abstract: A synergistic effect between Citric Acid (CA) and Microbial Phytase (MP) in improving phytate Phosphorus (P) utilization have been reported in broiler previously. In order to evaluate such additive effect in laying hen, an experiment was conducted using 224 Hyline-W 36 laying hen. Experiment began at 53 w of age and lasted in 64 w of age. The experiment was carried out using a completely randomized design with factorial arrangement (0 and 300 IU MP and 0, 20 and 40 g citric acid per kg of diet). Four replicate of 8 hens per each were fed dietary treatments including 1) Positive Control diet (PC) which meet NRC recommended available P level (0.3% available P), 2) Negative Control diet (NC) that was similar to PC diet except that available P was reduced by 0.2 %, 3) NC+300 IU microbial phytase per kg of diet, 4) NC+20 g CA per kg of diet, 5) NC+20 g CA+300 IU microbial Phytase per kg of diet, 6) NC+40 g CA per kg of diet, 7) NC+40 g CA+300 IU microbial Phytase per kg of diet. Dietary supplementation of MP to low available P diets significantly improved egg production and restored it to the level similar to PC group, but CA supplemented diets failed to create such effects. NC and diets with only CA supplementation had significantly lower feed intake compared to PC and MP supplemented diets. MP supplemented diets were used as efficiently as the PC diet, but CA had any effects on feed efficiency. Results obtained in our study suggests that contrary to the effects of CA in broiler chicks, CA couldn=t enhance phytase effectiveness in laying hen, probably due to high levels of Ca in laying hens diets.