Proteins are the most necessary nutrients for growth and surviva of fishes. Protein synthesis abilities of fishes are limited. Therefore, the maximum weight gain, wear out tissue renewal and to produce different products (products of protein like hormones, enzimes, intestine epitel cells) are fed by feed which contain their needed quality and levels protein. In fish feeding,utilization of unsuitable protein sources, above requirement protein rate or inadequate enegy level feed causes deamination of amino acids. Deamination of amino acids causes low weight gain, increase of production cost due to loses of protein which is the most costly part of the feed, moreover resulted the pollution on aquatic ecosystem due to excretion of ammonia-nitrogeneor urea-nitrogene. In this review, protein metabolism of fishes, excretion of ammonia, which is the last main product of protein and some factors that influence excretion of ammonia are summarized