Background: Gastric cancer is the fourth most common cancer in the world, the second most common cancer in Asia and the second leading cause of death among cancers in the worldwide, that the many factors are involved. Consumption of ω-3 fatty acids supplementation inhibits oxidative stress injury, prevention of cachexia , improve of appetite and reduce of the amount of matrix metaloproteinase enzyme activity in gastric cancer patients. The aim of this study is determination of the effect of ω-3 fatty acids supplementation on nutritional status, some inflammatory factors and matrix metaloproteinase in gastric cancer patients undergoing chemotherapy. Methods: In this study 15 volunteers patients with gastric cancer receiving chemotherapy for cases and 15 volunteers patients for controls are selected by Randomized allocation. Demographic, anthropometric factors, and nutritional information will be collected for patients and changes of weight, BMI, calories and nutrients intake will be calculated for both groups. 3 g of ω-3 fatty acid supplementation for intervention group and placebo for control group will be given for 45 days. Then blood samples in three stages (in the beginning, 30 and 45 days after interval) will be collected to test 2 and 9 matrix metaloproteinases, hepatic acute phase protein, malondialdehyde, tumor necrosis factor-ɑ, interlukin 1, 2 and 6, total antioxidant capacity, blood albumin, serum iron and blood fats.