Objective: To investigate the feasibility of using primary gross tumor volume (GTV) for predicting the pathologic complete response (pCR) to neoadjuvant chemoradiotherapy (nCRT) in patients with locally advanced rectal cancer (LARC). Methods: The data of 107 LARC patients undergoing nCRT followed by radical surgery in Department of Gastrointestinal Surgery, Xiangya Hospital of Central South University between March 2009 and December 2015 were reviewed. The factors for predicting pCR in LARC patients after nCRT were determined. The expressions of CD133 in the tumor specimens of these patients were measured by immunohistochemical staining, and then the relationship between primary GTV and rectal cancer stem cells was analyzed. Results: In the 107 LARC patients, pCR was achieved in 25 cases (23.36%). The primary GTV was positively correlated with the length along the longitudinal axis of the bowel (r=0.580, P<0.001) and maximal diameter (r=0.608, P<0.001) of the primary tumor, while the significant differences between pCR patients and non-pCR patients were only found in primary GTV (P=0.024), serum CEA level before nCRT (P=0.020), and multiple-drug combined chemotherapy (P=0.05). The optimal cut-off values for primary GTV to estimate the response of tumor was 70.29 cm3. The results of Logistic regression analysis showed that the small primary GTV (<70 cm3) (P=0.019) and multiple-drug combined chemotherapy (P=0.032) were independent promotion factors for LARC patients to achieve pCR after nCRT. The CD133 expression in the tumor tissues of patients with large primary GTV (≥70 cm3) was significantly higher than that in patients with small primary GTV (<70 cm3) (P=0.017). Conclusion: Primary GTV can be used as an independent predictive factor for pCR in LARC patients after nCRT. Big primary GTV is associated with low pCR rate, which is probably due to the bigger the primary GTV, the larger the amount of cancer stem cells in the tumor.