Abstract: Supplementing of ω-3 fatty acid has a large impact on anti-inflammatory and immune-modulating. These effects may be beneficial to cancer patients. In this study, the effects of ω-3 fatty acid intakes on inflammatory factors of gastric cancer patients were investigated during chemotherapy. This double blind clinical trial study was conducted on 30 adult patients (15 cases and 15 controls) with different pathologic type of gastric cancer during chemotherapy in Ardebil city, Iran 2010-2011. A 3 g of ω-3 fatty acid (1.8 g EPA and 1.2 g DHA per 10 g fish oil) and placebo were given for patient grouped randomly as omega and control. Blood samples were taken to measure IL-1β, IL-2, IL-6, hs-CRP and TNF-α at the beginning, 4 and 6 weeks after intervention. According to previous reports in current study, elevated serum concentrations of IL-1β, hs-CRP and TNF-α have been found in cancer patients with reduce in weight. While weight was significantly increased but IL-1β, IL-6 and TNF-α were decreased after consumption of ω-3 fatty acid, time dependently (p<0.05). Supplementation of ω-3 fatty acid can reduce the inflammatory effects and hence, it may have beneficial effects for gastric cancer patients during chemotherapy.