In the current investigation, a total of 110 Oreochromis niloticus were collected from ‎aquaculture farm at Sahl Eltina area, North Sinai Governorate suffering from skin and fin ulcers ‎with ascites, exophthalmia, and high mortalities. Aeromonas hydrophyla was isolated with a ‎prevalence rate of 40% and identified biochemically. A‏.‏‎ hydrophyla was used to induce ‎experimental infection to Nile Tilapia in which thyme and thymol were used as feed additive. ‎Evaluation of the protective effect of thyme and thymol as immunostimulant agents on fish ‎tissue against experimental infection with Aeromonas infection was studied. A total of ninety ‎apparently healthy fish tilapia were divided into three groups (GA, GB and GC of 30 fish per ‎group) in triplicate. GA (infected non treated group) was fed on basal diet. GB and GC were ‎fed on diet supplemented with Thyme vulgaris (10 g/ kg diet) and Thymol (3 g/ kg diet), ‎respectively for 8 weeks and then the groups were subjected to experimental infection with ‎Aeromonas hydrophyla isolate. The thyme and thymol groups had a nearly similar significant ‎immunostimulant effect than infected non treated group by using lysozyme activity assay.   ‎Sever hyperemia of gill filaments and gill arch, epithelial desquamation of the gill filament, ‎enteritis, necrosis and desquamation of epithelial cells of intestine, muscle necrosis, ‎myocardiolysis of heart, hydropic degeneration of liver and hemosiderosis of spleen were ‎observed in infected non treated  group. In the treated groups, the prophylactic effect of thyme ‎and thymol on fish tissues were clear against experimental infection with A. hydrophyla. These ‎results indicate that the using thyme or thymol as feed additive could enhance the immunity and ‎protect fish tissues from pathological deterioration induced by infection with Aeromonas ‎hydrophila which is considered as one the main fish pathogens in Egypt‎‎.