Transcatheter arterial chemoembolization (TACE) is often performed before radiofrequency ablation (RFA) for the treatment of early-stage hepatocellular carcinoma (HCC). TACE prior to RFA can expand the ablated area and reduce the tumor size, facilitating complete ablation. However, the factors correlated with size reduction remain uncertain. The aim of this study was to identify the factors associated with size reduction by TACE and develop a formula to predict the reduction rate. A total of 100 HCC patients treated with TACE followed by RFA at least 20 days later were enrolled. The tumor size was measured at the time of TACE and RFA, and correlations between the reduction rate and 13 clinical factors were examined. A formula to predict the reduction rate was built using the factors obtained by the analysis. Reduction in the tumor size was observed in 69 nodules, and the median reduction rate was 16.2%. A multivariate regression analysis revealed that a large tumor size (p< 0.01) and a long interval between the therapies (p= 0.01) were factors for a high tumor reduction rate, with tumor size more strongly related to the degree of reduction. A size reduction of more than 10% can be expected by waiting 20 days after TACE when the size of the tumor at TACE is over 25 mm in diameter. The tumor size