To investigate the effect of dietary organic acids on immune responses, gut microflora and phytate-phosphorus utilization in Leghorn laying hens, two experiments were conducted with Hy-line W-36 layers of 34 weeks-old. The birds were subjected to a 10-d adaptation period and main trial stage lasted 70 d. In the fist trial, a total of 210 hens were assigned to 7 dietary treatments with five replicates of six birds each to evaluate the effect of acid type and level on performance and immune responses. Dietary treatments were consisted a control group and levels of 0.25 and 0.5% of each of three different acids (salt forms of citric, butyric or fumaric acids). The results showed that dietary organic acid supplementation during first 35 d caused significant (P 0.01) increase in egg production. Also, egg production rate in whole experimental period was affected by acid type by level interaction so that, increase in acid level from 0.25 to 0.5% markedly (P 0.05) improved egg production only in citrate group. Egg mass was also affected (P 0.05) by dietary treatments during the first 35 d as well as whole trial period. Dietary inclusion of organic acids decreased serum cholesterol and triglycerides, and increased serum HDL (P 0.05). Antibody titers against Newcastle disease virus at days 7 and 14 post vaccine inoculation had different pattern among dietary treatments, and increase in dietary acid level totally decreased antibody titers at 7 th post vaccination. Experiment 2 was performed with 240 laying hens that randomly allocated to a 2 × 4 factorial arrangement of treatments with 2 levels of non-phytate phosphorus (60 or 100% of NRC recommendations) and 4 different basal diets (control versus diets containing salt forms of citrate, butyrate or fumarate). Egg weight was decreased (P 0.01) with diets low in phosphorus level during the first 35 d and in the whole trial period. Also, egg production was markedly (P 0.05) different among the acid diets so that only citrate resulted in significant increase in egg production rate. Feed conversion ratio was considerably (P 0.01) lower in 100% -diets than that of 60% -diets. Dietary inclusion of organic acids (P 0.01) and also increase in dietary non-phytate phosphorus level (P 0.05) resulted in significant decreases in serum cholesterol in mid and whole period, respectively. In contrast, serum HDL concentration was markedly (P 0.001) increased by dietary acid supplementation. Although Newcastle antibody titer wasn't affected by phosphorus level, dietary organic acids improved (P 0.01) antibody titers in both 7 and 14 d post inoculation. In overall, organic acids increased