It is difficult to know the effects of preoperative treatment on advanced rectal cancer even if using a variety of diagnostic imaging modalities. We report the efficacy of evaluating the therapeutic effect of using dual-energy CT(DECT)against rectal cancer after neoadjuvant chemotherapy(NAC). The subject sample consisted of1 1 patients who underwent rectal cancer surgery after NAC from September 2015 to January 2016. The pathological effective grade was higher if the after/before ratio ofcontrast enhancement on DECT was small(Grade 1a: 1.4, Grade 1b: 0.8, Grade 2: 0.4, Grade 3: 0.3). Therefore, a successful response rate occurred if blood flow was reduced after NAC. In this study, it was possible to predict the pathological response grade for rectal cancer via contrast enhancement using DECT.