Purpose: To investigate the use of Tm-170 seeds for brachytherapy in comparison to Iodine-125 seeds. Methods and materials: In vivo experiments were done in Balb/C mice. The KHJJ murine mammary carcinoma was selected as the tumor model because it is radio-resistant. KHJJ tumors were implanted on mice thighs and tumor growth was recorded as a function of time. When tumors reached the diameter of 7.5-9 mm, four groups of 20 tumor-bearing mice each, were implanted in the tumor as follows: 1. With Tm-170 seeds of activity 2.54 mCi, 2. With I-125 seeds of 0.5 mCi nominal activity (equal gamma dose to that of the Tm-170 seeds), 3. With control dummy seeds (empty titanium tubes of dimensions similar to Tm-170 seeds, that went through the same activation process as the real seeds) and 4. Control untreated mice. Results: The results show that control tumors and tumors implanted with the dummy seeds grew exponentially with time. Implantation of I-125 seeds resulted in the disappearance of 25% of the tumors (cured mice), 25% of the tumors grew slower than the control tumors (growth delay), and 50% of the tumors were not affected by the radiation. Implantation of Tm-170 seeds resulted in 60% cure, 20% growth delay and 20% of the tumors were not affected by the radiation. Conclusions: Tumors implanted with Tm-170 seeds were exposed to a smaller photon dose than those implanted with I- 125 seeds; nevertheless Tm-170 seeds yielded significantly better results. Brachytherapy with Tm-170 seeds in this experimental model is more efficient than with I-125 seeds since Tm-170 emits both beta and gamma radiation and its gamma radiation range is higher than that of I-125.