Zataria multiflora is well known to middle-east, Iran, Afghanistan and Pakistan where, it is cultivated only in warm areas. The objective of this article focused on this issue whether Zataria multiflora Boiss has effect on haematological and Growth parameters of Oncorhynchus mykiss. One hundred and twenty healthy finger ling fish, Oncorhynchus mykiss (mean weight 4.3 g) were commercially obtained and then transferred to the research field located in Tehran. Fish were fed on diets covering 0, 50, 100 and 150 mg kg^-1 Z. multiflora (Z.M.) as four groups; in triplicates. All of them were hand-fed (3-4% of body weight) to satiation 6-8 times a day for 8 weeks (60 days). There was no significant difference (p> 0.05) among the fish fed the different levels of Z.M for the mean weight. The maximum value of SGR was observed in group 50 ppm of Z.M. (2.85 ± 0.01) with significant difference (p< 0.05) with control (2.44 ± 0.0). Accordingly, this result showed that FCR value was minimum in control (7.20 ± 0.0) with no significant difference (p> 0.05) compared with group 50 ppm of Z.M. The result of WG for group 50 ppm of Z.M. (453.86 ± 0.46) showed a significant difference (p< 0.05) with those of other groups. On the other hand, Z.M. in concentration of 50 ppm could enhance WBC compared to control and other treatments as well as RBC but no difference was observed among the treatments (p> 0.05). Against, the values for Hb and MCH were significantly lower than others (p< 0.05). It is concluded that the fish fed with 50 ppm of Zataria multiflora increased mean weight of rainbow trout while enhancement of innate immunity significantly was occurred.