Trout fish is one of the most commonly used and market-friendly aquatic products, It is a rich source of protein and omega 3 long-chain polyunsaturated fatty acids. Deep frying under atmospheric condition is one of the usual methods of trout fish processing, resulting in quality and health loss of the fillet and frying oil. The use of vacuum condition during the frying process is one of the new technologies to improve the quality characteristics of oil and fried products. The purpose of this study is to modify a laboratory vacuum fryer and optimize the frying process of trout fish fillets under vacuum condition. According to the literature, the range of the effective parameters in vacuum condition were vacuum pressure (70-150 mmHg), oil temperature (90-130 °C), frying time (6-10 min), and de-oiling time (0-100 s). After the frying process, moisture loss, oil absorption, cooking loss, shrinkage, inner porosity, color, and textural properties of vacuum fried trout fish fillet were evaluated. Also, fried oil was assessed in terms of acid and peroxide value, malondialdehyde, color, and viscosity. To optimize the conditions for the vacuum frying process, response surface methodology (RSM) with central composite design (CCD) was used. Optimum condition was determined to be the vacuum pressure of 130 mmHg, oil temperature of 100 °C, frying time of 8.26 min, and de-oiling time of 25 s at the rotational speed of 400 rpm. validation test was conducted to confirm the model performance. According to the previous studies, the traditional frying process was also carried out at the oil temperature of 165 °C for 90 s, and de-oiling time similar to the optimum vacuum condition. The optimal vacuum and atmospheric frying process were compared. Results showed that during vacuum frying, moisture loss increased when the oil temperature and frying time increased, the oil absorption significantly affected by the vacuum pressure, oil temperature, frying time, and de-oiling time. Cooking loss of the fillets increased with the oil temperature and significantly affected by the de-oiling time. Shrinkage of the fillet did not significantly influenced by vacuum condition. The inner porosity of the fillet increases as the oil temperature and frying time increased and decreased by the increase in the vacuum pressure. Fillet color increased by oil temperature increasing and decrease by the de-oiling time increasing. Textural characteristics of the fillet increased by the increase of oil temperature, frying time, and de-oiling time. Frying oil properties including acid and peroxide value and viscosity did not significantly affected by vacuum condition. Malondialdehyde of the frying oil increased by oil temperature increasing and is significantly affected by the vacuum pressure. Oil color increased by oil temperature and frying time increasing. Finally, the fried fish fillets under optimum vacuum condition had compared to those fried under atmospheric condition, had significantly less moisture loss, oil absorption, cooking loss, inner porosity, redness and yellowness, hardness, and shear force. Frying oil used in optimum vacuum condition had significantly less malondialdehyde in comparison with that used in atmospheric condition. Keywords: Frying, Trout fish fillet, Vacuum and Atmospheric condition, Frying oil, Response Surface Methodology (RSM).