Objective To investigate the value of spiral CT in therapy observation during and after radiofrequency ablation (RFA) for lung or liver tumors. Methods Spiral CT-guided RFA were performed in 35 patients (38 lesions) with lung or liver tumors. CT value of lesions during operation and the changes of lesions after operation were observed. Results Thirty-eight lesions were accurately located under spiral CT. The position of the electrode and the expension of claw-like thin needle electrode could be observed directly. CT value of lesions decreased and some gasification foci in some lesions were observed after RAF therapy. One month after RFA, tumor volume decreased, and the tumors present as slightly low-density mass on contrast-enhanced CT. The total effective rate of RFA was 85.71% (30/35). Conclusion Spiral CT is able to accurately guide RFA treatment for lung or liver tumors and evaluate the efficacy of therapy.