The current study was carried out to investigate the effect of supplemental two levels of effective microorganisms (EM1) in the diet, as a biological treatment, on growth performance, digestibility coefficients, blood parameters and carcass traits of growing rabbits. Total of 45 growing rabbits (7 wk of age) were divided into three groups (15 in each) with five replicates for each group. Rabbits in all experimental groups were fed the same basal diet (20% CP and 16% CF); but differed in level of biological treatment. Rabbits in the 1^st group (control) were fed the basal diet without treatment, while those in the 2^nd and 3^rd groups were fed the basal diet treated with EM1 at levels of 1 (1% EM) and 1.5% (1.5% EM), respectively. Live body weight (LBW), digestibility coefficients (DC), some blood parameters and carcass traits were determined for up to 13 weeks of age. Results showed that 1 or 1.5% EM slightly increased contents of OM and NFE, and decreased EE and ash contents in diet of rabbits. Treatment with 1.5% EM increased CP and decreased CF contents, increased (P<0.05) final LBW and total weight gain, decreased (P>0.05) feed intake and improved (P<0.05) feed conversion ratio as compared to the controls. Digestibility coefficients of all nutrients improved by both levels of dietary EM treatments, being significant (P<0.05) only on CP, EE and NFE. Serum concentrations of total protein (TP), albumin (AL), globulin (GL) and glucose as well as activity of AST increased (P<0.05) were the highest (P<0.01), while activity of ALT was the lowest (P<0.05) in 1.5% EM treatment. Biological treatment with EM had no important effects on carcass traits and meat composition of rabbits. From the economic point of view, the observed nearly economic efficiency for both treated groups may suggest that EM at a level of 1.5% was effective in improving growth performance of rabbits without adversely effects on caecal digestion and healthy status of growing rabbits.