This study was conducted for the nutritional assessment of cancer patients undergoing radiotherapy (RT) and to investigate the changes in nutrition status, oral intake, morbidity and quality of life (QOL) in cancer patients after intensive nutrition counseling. Eighty-seven cancer patients were randomized to either a nutrition counseling group (n = 44, age 58.0 ± 2.2 years) or a control group (n = 43, 62.0 ± 1.8 years). Nutrition counseling accompanied RT, and the subjects received at least three sessions of individualized dietary counseling over the duration of RT. Assessment parameters were nutritional intake (24-h recall method), nutritional status Patient-Generated Subjective Global Assessment (PG-SGA), QOL and blood parameters including albumin. All parameters were measured at baseline, at the end of RT, and 1 month after the termination of RT. Body weight, body mass index (BMI), and energy and protein intake for the intervention and control groups did not differ significantly between baseline and the end of RT. However, at 1 month follow-up, protein intake was significantly decreased in the control group (p < 0.05). Blood albumin, total protein (TP), total lymphocyte count (TLC) were not different between the two groups. According to PG-SGA stage, at 1 month follow-up, patients in the intervention group showed increased number of patients with stage A status (well nourished). In addition, insomnia and nausea was significantly improved in the intervention group assessed by QOL. We suggest that repetitive and intensive nutritional counseling is necessary to improve QOL and to prevent deterioration of nutritional status in cancer patients receiving RT.