Relative dose intensity (RDI) in chemotherapy represents an affecting factor on survival. Body composition is the basic information and the test can be easily evaluated. The aim of the study was to investigate the relevance between RDI and body composition. We conducted a prospective cohort study of 51 patients (31 of Breast/20 of colon cancer) who were candidate of adjuvant chemotherapy. All of them were obtained informed consent and were conducted body composition test before initiation of adjuvant chemotherapy. The body composition was measured using bioelectrical impedance analysis with an InBody720 (Biospace Co.). The variables included body mass index (BMI), total muscle mass, body weight (BW), skeletal muscle mass, skeletal muscle mass/BW, total muscle mass/BW and body fat mass/BW. The correlations of RDI and variables in body composition were determined using Pearson correlation coefficient analysis. In breast cancer, RDI had a significant correlation with BMI(r of 0.373, p＝0.039), skeletal muscle mass/BW (r of 0.582, p＝0.001), total muscle mass/BW (r of 0.526, p＝0.002), body fat mass/BW (r of -0.558, p＜0.001) and total skeletal muscle mass (r of 0.538, p＝0.002). In colon cancer, RDI had a significant correlation with total muscle mass/BW (r of 0.562, p＝0.010) and body fat mass/BW (r of -0.546, p＝0.013) High total muscle mass/BW and low body fat mass/BW were a positive correlated with RDI in breast cancer and colon cancer with adjuvant setting. The body composition may be a potential parameter to predict a prognosis in cancer chemotherapy.