The present study was done to determine the effects of exogenous β-mannanase (Hemicel® enzyme) and methionine on growth performance and blood parameters of Nile tilapia Oreochromis niloticus fed plant protein diet. Two basal diets were formulated to contain 27% CP one of them contained fishmeal (A) and other was plan protein (B). This study included six treatments were as the following: T1: (negative control) fish fed with diet A, T2: (positive control) fish fed on diet B, T3 and T4: fish fed on diet B with adding 0.025 and 0.05 % of Hemicel®, respectively, and T5: fish fed the diet B with 0.025% of Hemicel® and 5% methionine and T6: Diet B with adding 0.05 % Hemicel and 5% methionine. 120 juveniles were randomly divided into 12 plastic tanks (water size: 35 liters) with density of 10 fish/tank. Fish reared in saline of 3ppt, and fed two times daily for six days weekly with rate of 4% of body weight. Results confirmed that feed additives of Hemicel and Dl-methionine significantly improved nutritional utilization of plant protein diets. One way ANOVA analysis did not show significant differences in growth performance between fish fed with a soy-based diet containing 0.025% Hemicel® and Dl-methionine 0.5% (T5) and those fed with the diet containing fish meal (T1). Fish in T5 and T4 had highest body content of protein and the best blood indicator. Generally it can be totally replaced fish meal by soy bean with adding Hemicel® and Dl-methionine at levels of 0.025% and 0.5% respectively.