Background: Disease-related malnutrition, as a major cause of morbidity and mortality in cancer patients, is very common in patients with gastric cancer (GC). Malnutrition in cancer patients was reported to have important adverse effects, including a decreased response and tolerance to treatment, a decrease of performance, shorter survival, and lower quality of life. The treatment approaches involving chemotherapy is known to develop various acute and chronic symptoms that restrict eating and, thereby, exert a profound impact on nutritional status. Method: In this study, 82 patients with GC with an average age of 48.33 ± 10.74 were enrolled. Patients were followed up for 6 months without any nutritional intervention and/or education. A 168 item semi-quantities food frequency questioner was completed by a trained nutritionist at the beginning of the study and six months after the start of chemotherapy. Results: Intake of vitamin A (T0:585.52 ± 203.34 vs. T6:529.48 ± 138.91, t = 2.96), Thiamin (T0:2.09 ± 0.76 vs. T6:1.80 ± 0.72, t = 2.81), vitamin B6 (T0:2.03 ± 0.53 vs. T6:2.29 ± 0.73, t = 2.56), and vitamin B12 (T0:5.79 ± 3.96 vs. T6:4.48 ± 2.20, t = 2.43) significantly decreased after 6 months of receiving chemotherapy. On the other intake of beef (T0:17.79 ± 25.48 vs. T6:12.58 ± 16.66, t = 2.06), low-fat milk (T0:52.57 ± 69.80 vs. T6:29.18 ± 45.89, t = 2.95), cream (T0:2.42 ± 4.16 vs. T6:1.06 ± 1.68, t = 2.88), and raw vegetable (T0:6.54 ± 9.55 vs. T6:3.85 ± 5.23, t = 2.54) significantly decreased. Conclusion: Nutritional deterioration is an important part of the pathogenesis of cancer and its treatment that can occur at any point in the timeline of cancer diagnosis, treatment or support. Therefore nutritional counseling and supportive services are needed for cancer patients, especially when their disease is diagnosed.