Purpose: To evaluate the ablation ranges and temperature rising rates of three microwave antenna with different diameters in vitro bovine and in vivo porcine livers. Materials and Methods: In vitro and in vivo study, microwave ablations (MWA) were induced at a power output of 70 W for 5, 8, and 10 min; a total of 54 and 36 ablations were performed, respectively. Temperature measurements were performed in the 70 W/10 min group with 3 different microwave antennas. Results: In vitro study, the long axis diameter for the 16G microwave antenna was longer than the 15G (P < 0.05) and 14G microwave antennae (P < 0.05). The sphericity index for the 14G and 15G microwave antennae was higher than the 16G microwave antenna (P < 0.05 for both). In the in vivo study, the short-and long-axis diameters, and sphericity index were not statistically different between the 14G, 15G, and 16G groups. The temperature data were not statistically different in both studies. Conclusions: Although the thin microwave antenna had a longer long-axis diameter and a lower sphericity index in vitro study, the thin microwave antenna achieved the same ablation effect in vivo application. MWA has several advantages, and the thin microwave antenna decreases the puncture risk and complication rate. Thus, a thin microwave antenna (15G and 16G) is beneficial in clinical applications and will have a significant influence in the development of MWA.