In this study, the effect of different concentrations of three gas mixtures (carbon dioxide, nitrogen, oxygen), as well as vacuum conditions, and ordinary packagings in 3 types  of flexible 4-layer films, containing fennel essential oil were evaluated for extending shelf life of fresh rainbow trout fish at 4⁰C. Ordinary conditions as control samples, were compared with four kinds of modified atmosphere packaging 1)75% CO₂+25% N₂, 2) 25% CO₂+75% N₂, 3)% 48CO₂+48% N₂+4% O₂ and 4) under vacuum condition, along with injection fennel essential oil(2% WE/Ws), and samples without essential oil. Fish samples were packaged in different flexible multi-layer films, PET12/AL7‌‌/OPP20/‌‌LLD90, PET12‌‌/AL7/PET 12/LLD‌‌100 ‌‌, and‌‌ PET12/AL7‌‌/PA15/‌‌LLD‌‌90, under MAP. The microbial aerobics and anaerobic counts and the pH changes, determination of peroxide content, total volatile nitrogen, and sensory properties of samples were performed on the packaged samples at different 5, 10, 15, 20 days. The experiments revealed that the gas composition of 75% CO₂ combined with fennel essential oil due to high antimicrobial properties of these factors, and 4-layer PET12/AL7 /PA15/ LLD 90)123 μ) ‌‌‌‌along with poly amid as a low water vapor transmission rate containers in spite of less thickness, were used to maintain the long-term of (20 days) fresh trout fish and had a desire effect on controlling microbial growth and chemical properties, and sensory evaluation of samples(odor, color, texture, appearance, and taste) were observed better.