The aim of this study was to evaluate the growth-promoting effect and safety of genetically modified Chlorella vulgaris expressing olive flounder (Paralichthys olivaceus) growth hormone(fGH) when used as a feed additive for olive flounder. At the start of the experiment, 252 juveniles with an initial body weight averaging 41.9±0.31g (mean±SEM) were randomly distributed into 21 rectangular tanks (12 fish per tank; N = 3 tanks per treatment). A total of seven experimental diets were prepared, including no addition of fGH (CON), 0.25 (fGH0.25), 0.5 (fGH0.5), 0.75 (fGH0.75), and 1% of fGH (fGH1.0) in a basal diet as well as 0.5 (W0.5) and 1% (W1.0) of wild-type Chlorella vulgaris as sham diets. The fish were fed two times (9:30 and 16:30 h) per day to apparent satiation for 8 weeks. At the end of the feeding trial, no significant difference in growth performance indices such as weight gain, specific growth rate, feed efficiency, and survival rate as well as morphological changes, including condition factor, hepatosomatic index, and viscerosomatic index among the tested diets was detected (P > 0.05). Plasma metabolites, including glutamic oxaloacetic transaminase, glutamic pyruvic transaminase, glucose, total protein, triglyceride, and total cholesterol were not significantly affected by the tested diets. Antioxidant activities and immune responses as well as expression levels of genes involved in the GH/IGF-axis and nutrient uptake will be discussed later.