Objective To study the characteristics of microscopic spread of esophageal squamous-cell carcinoma (ESCC) and the influence of clinicopathological features on it to help define the clinical target volume (CTV) margin in radiotherapy.Methods Sixty-four surgical specimens of ESCC were observed for longitudinal microscopic spread per centimeter both proximally and distally from the tumor.The shrinkage ratio of each specimen was calculated and used for tissue incision.Results The further the distance beyond the tumor, the lower the incidence there was of microscopic spread.Positive rates of microscopic spread in group 3 cm of proximal and distal were 4.8% and 6.9%, respectively, and in group 4 cm were both 3.6%.Tumors longer than 5 cm in length,with poorer differentiation, lymph nodes metastasis and more aggressive phase had higher positive rates (79.3% vs 45.7%,77.4% vs 45.5%,76.0% vs 51.2%,70.5% vs 40.0%,χ2=7.52,6.86,3.91,5.36;P=0.006,0.009,0.042,0.021).Differentiation and tumor length were main factors contributing to microscopic spread (χ2=0.19,4.82;P=0.020,0.017).Conclusions To cover 95% of the microscopic spread,a margin of 3.0 cm proximal and 4.0 cm distal beyond gross tumor volume is needed and as to 90%, a margin of 3.0 cm both proximal and distal is needed.Moreover, the influence of pathological features should be taken into account.