Abstract: Objective To evaluate the relationship between serum thymidine kinase 1(TK1) and esophageal squamous cell carcinoma (ESCC) treated with radiotherapy. Methods Via the enzyme-linked immunosorbent assay (ELISA) for the detection of the TK1 levels in 50 healthy cases and 50 ESCC cases, we analyzed the difference between the two groups. The positive rate of TK1 in ESCC cases was detected, and the relationship with TNM staging, differentiation degree, length of lesion and type of angiography was evaluated. The TK1 level variation of 50 patients with ESCC were compared before and after 1 week and 1 month treatment. The relationships between TK1 and radiotherapy Dose, treatment modalities and clinical therapeutic efficacy were evaluated. Results The TK1 (pmol/L) concentration in ESCC group was 3.14±1.70, and the healthy group was 1.29±0.66, P<0.05. The positive rates of serum TK1 in well differentiated group, moderately differentiated group, low undifferentiated group were 36.4%, 84.4% and 71.4% respectively, P<0.05. In stage Ⅱ, Ⅲ and Ⅳ, they were 62.5%, 64.9% and 40.0%, P>0.05. The cases with lesion length >6 cm and ≤ 6 cm accounted for 63.9% and 57.1%, P>0.05. And medulla type, ulcer type, narrow type and cavity type cases were 68.7%, 57.1%, 62.5%, 66.6% (P>0.05). The concentrations of TK1 (pmol/L) in the groups with concurrent chemoradiotherapy and the radiotherapy were 2.35±0.86, 1.81±0.57, 1.13±0.46, 2.60±1.43, 1.59±0.64, 1.10±0.56 (P<0.05) before treatment, 1 week and 1 month after the end of radiotherapy. In the groups of 54-59 Gy and 60-66 Gy, TK1(pmol/L) levels were 2.33±0.91, 1.83±0.55, 1.07±0.33, and 2.57±1.34, 1.63±0.63,1.12±0.57 respectively, P>0.05. In the CR, PR and SD groups TK1 (pmol/L) were 2.33±0.96, 1.84±0.57, 1.03±0.32; 4.89±1.37, 1.64±0.65, 1.13±0.59; 2.63±0.51, 1.44±0.19 and 1.22±0.32, respectively, P>0.05. Serum TK1 concentrations in each group before treatment, after 1 week, 1 month after treatment showed no significant differences (P>0.05). Conclusion The monitoring of TK1 levels has significance in the clinical diagnosis and treatment of ESCC.