A histopathological health assessment for olive flounder Paralicthyes olivaceus, with different size based on total body length and weight, was carried out. Two samplings were performed: (I) on 4th October 2010 and (II) on 25th January 2011. Two groups of fishes were administered for the study from the same culture system and age group- the big sized fishes (fast growing) and the small sized fishes (slow growing). Clinical record, blood chemistry, total protein (Tp), hemoglobin (Hb), hematocrit (Ht), functional indices i.e., hepato somatic index (HSI) and hepato hypertropic index (HHI), and histological studies of different organs (liver, stomach, intestine, spleen, kidney and heart) were performed for each individual of each group. Moderate to severe fatty change of liver were observed in both groups of fishes in case of sampling I. Moderate to severe atrophy of liver were observed in both groups of sampling II. Focal epicarditis were observed in sampling I for big sized fishes, and vacuolative changes of gastric gland of big sized fishes were observed in sampling II under light microscopic examination. Tp, Ht, and HHI were recorded significantly higher (p values ranged from 0.001 to 0.032) for big sized fish group in sampling I. In sampling II only HHI was found significantly higher (p = 0.004) in big sized fish group. Thus, it can be concluded that fast growing individuals are more vulnerable to disease based on the histopathological assessment. This information will be helpful to prevent and control diseases for different age groups of olive flounder by taking different management practices.