Objective To investigate the changes of regulatory T cells (Treg) in patients with hepatocellular carcinoma (HCC) after ultrasound- guided percutaneous cool- tip radiofrequency ablation (RFA), and to discuss its influence on the prognosis. Methods A total of 30 patients with HCC were enrolled in this study. The percentage of Treg in peripheral blood was estimated with flow cytometry before RFA and one, 4, 7 and 12 months after RFA. During the follow-up period, the therapeutic effects were evaluated by contrast enhanced sonography or contrast enhanced CT scanning. By using the methods of receiver operating characteristic (ROC) curve and Kaplan-Meier survival function, the correlation of Treg dynamic changes with the progression-free survival time was analyzed. Results One month after RTA, the tumor response (TR) rate in the 30 patients was 93.3% (28/30), the tumor progression (TP) rate was 6.67%(2/30). The percentage of Treg before RFA was (9.42 ± 1.16)%, which decreased to (6.55 ± 0.97)% one month after RFA, the difference was statistically significant (t = 15.325, P <0.001). Twelve months after RFA, TR rate became 33.3%(10/30), and TP rate became 66.7%(20/30). The preoperative percentage of Treg of TR group was (8.75 ± 0.72)%, which was significantly lower than that of TP group (9.76 ± 1.20)%, the difference was statistically significant (t=-2.448, P=0.021). ROC curves indicated that the optimal cut-off value of Treg nadir was 4.82%, the sensitivity was 90.0% and the specificity was 60.0%. The optimal cut-off time to reach Treg nadir was 5.5 months, the sensitivity was 70.0% and the specificity was 85.0%. Kaplan-Meier curve analysis showed that after RFA the progression-free survival rate (PFS) of patients with Treg nadir ≤ 4.82% was significantly higher than that of patients with Treg nadir>4.82%. PFS of patients with reaching Treg nadir≥5.5 months was significantly higher than that of patients with reaching Treg nadir<5.5 months. Log-rank test results were字2=5.207, P=0.023; 字2=22.079, P < 0.001, respectively. Conclusion Percutaneous cool-tip radiofrequency ablation can decrease the percentage of Treg cells. Besides, Treg nadir and the time reaching Treg nadir can reflect the prognosis of HCC patients after RFA to a certain extent.