Background & Aim: By increasing resistance of fish pathogens to chemical drugs, using herbal plants with antimicrobial properties is necessary.In this study, the composition of the essential oil and antimicrobial activity of aerial parts of Achillea wilhelmsii C.Koch growing wild in Zagross mountain (Southwest of Iran) was analyzed against common bacterial and fungal pathogens of fish. Experimental: In the present study, aerial parts of A. Wilhelmsii were tested against five bacterial strains including Streptococcus iniae, Yersinia ruckeri, Vibrio angularum, Aeromonas hydrophila and Pseudomonas aeruginosa and four pathogenic fungi namely Saprolegnia sp., Fusarium solani, Candida albicansand Aspergillus flavus. Also, the oil constituents of A. Wilhelmsii were analyzed by gas chromatography/mass spectrometry (GC/MS). Results & Discussion: The major composition of the essential oil of A. Wilhelmsii was 1.8-cineol (25.2%), Camphor (18.9%), Linalool (6.9%), Borneol (5.7%) and Artemisia alcohol (4.3). All microorganisms showed more sensitivity to essential oil of A. Wilhelmsii than the control antibiotics. Maximum antibacterial and antifungal activity was observed against Y. ruckeri (25.4 mm)and C. albicans (25 mm) respectively, while S. iniae (16.4 mm)and A. flavus (14 mm)exhibited the least sensitivity. In addition, MIC test showed that minimum concentrations of the essential oil ranged from 125 to 800 μg/l were able to inhibit the growth of the selected bacterial and fungal pathogens. Industrial and practical recommendations: The results showed that the essential oil of A. Wilhelmsii could be a potential source of new and effective antibacterial components in aquaculture industry. Although, more study needed to purify, fractional and characterize various antimicrobial compounds from the essential oil of A. Wilhelmsii. پرونده مقاله