New and simple interstitial antennas tipped with reactive load (RL) are presented. For their design, a matching technique based on transmission line theory is also suggested. To verify the technique, one immersed in muscle phantom is designed, fabricated, measured, and compared. The measured reflection coefficient at 2.45 GHz is -28.4 dB, which considered the best among those reported, and the measured region greater than 43 degrees C is a rugby ball (major axis 4.5 cm And minor axis 2.3 cm), which demonstrates that it can be applied for a deep seated tumor or cancer.