This study was done on 200 Nile tilapia (Oreochromis niloticus)  of different ages were collected from farms of Central Laboratory for Aquaculture Research (Abbassa, Abou Hammad, Sharkia, Egypt) during period from late November, 2014 to march, 2016. The collected fish samples were clinically diseased showed, respiratory manifestation; ulcer and hemorrhages on body surface, cotton wool like tufts on skin, fins and in eye caused opacity and detached scale. Internally the affected fish showed enlarged pale or yellow, many cases showed congested spleen and kidney, intestine free from any food particles and gall bladder enlarged and distended with bile. The Mycological examination by different method, eleven different fungal isolates were macro and microscopically  identified, Saprolegnia spp., Asp. niger, Asp. flavus, Asp. ochraceus, Asp. terreus , Asp.versicolor, Zygomycets group, Penicillium spp., Fusarum spp., Cladosporium spp. and Candida albicans. The study revealed that Asp. niger was the most fungus isolated and affected fish during summer season, while Saprolegnia spp. was the most fungus affected fish and isolated during winter season . Saprolegnia spp. responsible for high percentage of mortality among affected fish and also lead to decrease percentage of hatchability among affecting fish eggs. This study revealed that row garlic prevents growth of Asp. niger, Asp. flavus and Cladosporium spp. in vitro.