Purpose: To retrospectively evaluate risk factors related to incomplete computed tomography (CT)-guided radiofrequency (RF) ablation of metastatic and primary lung tumors. Materials and Methods: This study included 93 patients with 147 tumors: 70 men, 23 women; median age 54 y (range, 19-81 y); 24 cases of primary lung tumors, 69 cases of metastases; average largest diameter of tumors, 1.8 cm 1.2 (range, 0.3-6.0 cm). Local efficacy was evaluated based on CT follow-up scans. Complete ablation rates (CARs) for tumors were calculated according to several variables; independent risk factors for local tumor progression (LTP) were examined by binary logistic regression analysis. Results: CAR of tumors was 60.54% within first 6 months after lung RF ablation; median interval of LTP was 1.5 months (mean, 1.3 months +/- 1.0; range, 0 days to 3 months). Compared with tumors > 3 cm, CAR of tumors <= 3 cm was significantly higher (68.55% vs 17.39%, P < .001). CAR of tumors with complete ablation margin (AM) was dramatically higher compared with tumors with incomplete AM (74.77% vs 16.67%, P <.001). Among tumors with complete AM, CAR of tumors with shortest distance between outer edge of tumor and AM (ablative margin D) >= 5 mm was compared with tumors with ablative margin D 1-4 mm (85.96% vs 62.96%, P = .005). Multivariate regression analysis showed that lobulation and/or spicules, contact with blood vessels, and ablative margin D < 5 mm were independent risk factors for incomplete lung RF ablation. LTP was likely to develop at the edge of ablated lesions and especially the site of incomplete AM or shortest AM. Conclusions: RF ablation for lung cancers should be individualized based on tumor size, morphology, and tumor type to obtain an adequate AM.