Patients with colorectal cancer may experience symptoms such as diarrhea, nausea, and anorexia, during surgery and chemotherapy, which can increase the risk of malnutrition. In addition, dietary habits play a key role in the onset of colorectal cancer; therefore, it is necessary to improve dietary habits to prevent recurrence during treatment after diagnosis. In this study, a clinical nutritionist conducted 4 interviews for patients diagnosed with colorectal cancer and scheduled for colectomy: before surgery, after surgery, 1st chemotherapy, and 2nd chemotherapy, and provided nutrition care for each treatment course to determine its effects on nutrition status and disease prognosis. Significant weight loss but no decrease in muscle mass was observed during treatment. Body fat mass, although not statistically significant, showed a decreasing tendency. The percentage of people who responded ‘yes’ to the below items increased after compared to before receiving nutrition education: ‘I eat meat or eggs more than 5 times a week,’ ‘I eat seafood at least three times a week,’ ‘I eat vegetables at every meal,’ ‘I eat fruits every day,’ and ‘I eat milk or dairy products every day.’ These results indicate that the patients changed their dietary habit from a monotonous eating pattern to a pattern of consuming various food groups after receiving nutrition education. These results suggest that continuous nutrition care by clinical dietitians, according to the patient’s treatment process, can help improve the patient's nutritional status and establish healthy eating habits.