This study was carried out to investigate the effects of the inoculated diet with Paecilomyces japonica on weight gain, feed efficiency and cordycepin contents in the breast and thigh meats of broilers. One hundred and twenty broiler chicks were divided into four dietary groups ; 1) control (basal diet), 2) T1 (substituted with 5% inoculated diet), 3) T2 (substituted with 10% inoculated diet) 4) T3 (substituted with 20% inoculated diet) during 4 weeks. Cordycepin contents of inoculated diet with Pacilomyces japonica was significantly higher than before inoculation (P<0.05). Body weight gain and feed efficiency were highest in T1 group when fed starter diet (P<0.05) but these were lowest in T3 group fed when fed finished diet (P<0.05). Body weight gain and feed efficiency were the lowest in T2 group for overall period than other groups. Cordycepin contents of breast and thigh meats were significantly higher in Pacilomyces japonica groups than control at 5 weeks of age (P<0.05). However, it was similar among the T1, T2 and T3 groups. These results indicated that body weight gain and feed efficiency tended to increase in birds fed replaced 5% inoculation diet Cordycepin contents of breast and thigh meats would be increased significantly in birds fed inoculated diet (P<0.05).