Introduction: Gastric cancer is the fourth common cancer and the second leading cause of death from cancer worldwide. Omega fatty acids are a group of non-saturated fatty acids of long chains derive from linolenic, linoleic and oleic fatty acids. The current study aimed at examining the possible effects of oral administration of omega-3, -6 and -9 fatty acids on gastric cancer cell apoptosis in patients with gastric adenocarcinoma via the evaluation of BCL2 gene and protein expression. Methods: In the present case-control study, a total of 34 patients with gastric cancer referring to the Endoscopy Clinic at Tabriz University of Medical Sciences were selected and divided into 2 groups (n = 17). In group 1, patients were treated by platinum drugs without any supplementation and in group 2, patients were treated by platinum drugs supplemented with omega-3, -6 and -9 fatty acids over 3 courses, which each lasted 3 weeks. After stomach biopsy, RNA extraction, cDNA synthesis and real-time polymerase chain reaction (RT-PCR) were performed. Results:The results showed that Bcl-2 gene and BCL2 protein expression in patients who received omega-3, -6 and -9 fatty acids with chemotherapy was lower than those of the ones who not received omega fatty acids. Also, there was a significant decrease in Bcl-2 gene expression (P < 0.05). Conclusions:The oral administration of omega-3, -6 and -9 fatty acids with the chemotherapeutic drug cis-platinum in patients with gastric adenocarcinoma could decrease Bcl-2 gene expression and BCL2 protein production in the cancerous gastric tissue.