Catheter contact force is well known as one of the factors which has a strong positive correlation with lesion formation. We hypothesized that the catheter contact force substantially affects the ablation impedance variation during radiofrequency (RF) catheter cardiac ablation, due to its direct effect on the electrical current output from the RF generator. However, such a hypothesis needs to be validated through further investigation. Therefore, the purpose of this study was to understand the effect of the catheter contact force related to the contact angle on the ablation impedance in the radiofrequency catheter cardiac ablation process. Thus, we developed a special in vitro experimental system that enables to precisely set the catheter contact angle to the heart tissue's surface. The catheter ablation test was held at five different contact angles to the heart tissue surface. A RF current was delivered at each angle at contact force values of 2, 4, 6, 10, 15, 20, 30, and 40 gf. RF catheter ablation examination was repeated six times each for the five levels of contact angles and eight levels of contact forces. Then, the maximum impedance and average impedance during the ablation process for each condition were recorded. The results revealed that the catheter contact force has a significant correlation with ablation impedance. When the contact force was increased, the different impedance between the maximum impedance and average impedance also increased. The percentage of contact area also has a significant correlation with the ablation impedance. When the percentage of contact area was increased, the different impedance between the maximum and the average impedance also increased. However, the ablation impedance did not have a significant difference with each catheter contact angle. The present study showed a significant impact of the catheter contact force on the ablation impedance in RF catheter ablation systems.