To elucidate the effect of feeding differently prepared CaCO₃ (industrially purified limestone) on layingperformance, egg shell quality and serum calcium and phosphorus concentration, totally 288 layers (52 wksold), with 16 birds per replicate, 3 replicates per treatment for six treatments were employed into the seriesof CaCO₃ treatments followed by feeding study. Six CaCO₃ treatment and diet preparation includes mashdiet with naturally sized CaCO₃ (MNaC), mash diet with coarsely particled CaCO₃ (MCoC), mash diet withfinely particled CaCO₃ (MFiC), pelleted diet with naturally sized CaCO₃ (PNaC), pelleted diet with caseincoated CaCO₃ (PCnC) and pelleted diet with agar coated CaCO₃ (PAgC). Hen day egg production of the birdfed both fine CaCO₃ and pelleted CaCO₃ were lower (p<0.05) than those fed other CaCO₃ diets. Althoughthere is no difference in average egg weight among treatments, the weight tended to be heavier once the eggproduction rate was low. There is no difference in feed conversion ratio (intake/egg) among treatments. Eggshell thickness was the highest (p<0.05) with feeding of MCoC whereas the value was the lowest with feedingof pelleted CaCO₃ diet. Although there is some difference in egg shell thickness, the shell breaking forces werenot significantly different among treatments. Calcium content of the shell was the lowest (p<0.05) in the eggfrom the layer fed finely particled CaCO₃. Serum Ca content tended to decrease up to 8 hrs after feeding,then tended to increase afterwards. Although both agar and casein coating of CaCO₃ tended to retard therecovering increase of blood Ca, the difference needed to be clarified with individual ovulation cycle. SerumP level was generally decreased as time passed after feeding. Crude protein utilizability was the highest withfeeding of agar coated CaCO₃ and that of fat was higher with hens fed pelleted diets. Both ash and Putilizabilities of diet with finely particled CaCO₃ were the highest among treatment although there were nodifferences in both energy and Ca utilizabilities. Conclusively, both the particle size of CaCO₃ and pelletingof CaCO₃ diet could affect shell quality of the egg. The effect was more evident by the differentiation ofCaCO₃ particle size than any other processing such as pelleting and coating.