ObjectiveTo investigate effect of carbon nanoparticles on number of lymph nodes harvested in radical operation of rectal cancer after neoadjuvant chemoradiotherapy. MethodsOne hundred and five patients diagnosed with low and middle rectal cancer and received radical operation after neoadjuvant chemoradiotherapy in Cancer Hospital, Chinese Academy of Medical Sciences from March 1, 2014 to October 31, 2015 were included. Thirtysix patients were injected with carbon nanoparticles by colonoscopy before surgery and were classified as study group, and the rest patients were classified as control group. According to the same principle of surgery and procedure of pathological specimen handling, the effect of carbon nanoparticles on lymph node harvested in resected specimens was analyzed. ResultsThe total lymph nodes harvested were 764 in the study group and 1 242 in the control group. Among them, the metastatic lymph nodes were 19 in the study group, 58 in the control group. Although the average lymph nodes harvested in each patient had no significant difference between the study group and the control group (21.22±7.13 versus 18.00±9.84, t=1.739, P=0.085), the proportion of patients with 12 or more than 12 lymph nodes harvested in the study group was significantly higher than that in the control group [88.9% (32/36) versus 71.0% (49/69), χ2=4.287, P=0.038]. The ratio of patients with metastatic lymph nodes [27.8% (10/36) versus 33.3% (23/69), χ2=0.339, P=0.561] and the average metastatic lymph nodes harvested in each patient (1.90±1.29 versus 2.52±2.33, t=0.788, P=0.437) all had no significant differences between the study group and the control group. ConclusionThe injection of carbon nanoparticles by colonoscopy before surgery could increase detection rate of 12 or more than 12 lymph nodes in resected specimens of patients who were diagnosed with low and middle rectal cancer and received radical operation after neoadjuvant chemoradiotherapy.