A study was carried out to examine the effect of different levels of Herbal methionine and DL-methionine on the growth performance and immunocompetence of broiler chickens fed on a corn-soybean meal diet. A total of 225 old male broiler chickens (Ross 308) were used as experimental samples in a completely randomized design. They were randomly assigned to 9 treatments each of 5 replicates in every experimental unit (cage). Treatments included basal diet supplemented increasingly with Herbal methionine and DL-methionine during three growth periods of the birds. Treatments included: T1: basal diet without any additional methionine, T2: basal diet plus 0.07, 0.06, 0.05, T3: basal diet plus 0.15, 0.11, 0.10, T4: basal diet plus 0.22, 0.17, 0.14, T5: basal diet plus 0.29, 0.23, 0.19 % DL-methionine for starter, grower and finisher periods respectively, T6: basal diet plus 0.07, 0.06, 0.05, T7: basal diet plus 0.15, 0.11, 0.10, T8: basal diet plus 0.22, 0.17, 0.14, T9: basal diet plus 0.29, 0.23, 0.19 % Herbal methionine for starter, grower and finisher periods respectively. Feed intake and body weights were recorded during the three experimental periods and then, feed conversion ratios calculated. To investigate the immunocompetence of birds at 28 and 42 days of age, the following two tests were carried out: 1) response to antibody titer against antigen of Sheep Red Blood Cells (SRBC) and 2) response to dilution of 1-choloro 2-3- dinitrobenzene (DNCB). The results revealed that the maximum finisher body weight gain was related to the two highest levels of DL-methionine and Herbal methionine with significant differences as compared with the other treatments, while the highest level of DL-methionine causing minimum finisher body weight gain. Birds fed the basal diet plus 0.15, 0.11, 0.10 % DL-methionine for starter, grower and finisher periods respectively and also, the basal diet plus 0.22, 0.17, 0.14 % Herbal-methionine for starter, grower and finisher periods respectively, exhibited the most desirable performance and immunocompetence. Thus, according to the obtained results, it seems that Herbal methionine could in effect replace DL-methionine.