Purpose: Tc-99m-MIBI (MIBI) and Tc-99m-Tetrofosmin (TF) are commonly used for scintimammography (SMM). We compared the diagnostic ability of SMM using Tc-99m-MIBI and Tc-99m-TF for the differential diagnosis of breast mass. Materials and Methods: The study subjects were comprised of 123 breast lesions and 86 normal breasts of 114 patients who underwent SMM. Bilateral prone images and anterior supine images were obtained at 5 minutes and 1 or 3 hours after intravenous injection of 740 MBq of either MIBI or TF. Sizes of tumors were not significantly different between the MIBI and TF groups. First, two observers independently read the SMM without clinical information (1st interpretation), then read again with information about mass location (2nd interpretation). Sensitivity and specificity of each radiopharmaceutical for the diagnosis of breast cancer were evaluated in terms of image acquisition time, tumor size, and location. Results: The SMM showed a good agreement between two observers for 1st and 2nd interpretation, except for TF SMM at 3 hr. For the first interpretation, the sensitivities at 5 min, 1 hr, and 3 hr were not significantly different between MIBI and TF SMM (81.6%, 80.0%, 60.9% in MIBI vs. 88.9%, 80.6%, 42.9% in TF), although the senstivities of 3 hr images were significantly lower than 5 min images in both MIBI and TF SMM. The specificity of TF at 5 min was superior to that of MIBI (81.5%, 90.0%, 82.9% in MIBI vs. 96.7%, 100%, 90.0% in TF, p<0.01 MIBI vs. TF at 5 min). For the second interpretation with information of mass location, the sensitivities at 3 hr images were significantly lower than 5 min images (86.8%, 86.7%, 78.3% in MIBI vs. 88.9%, 93.5%, 57.1% in TF) between MIBI and TF SMM. However, there was no significant difference in the specificity (60.0%, 53.8%, 75.0% for MIBI vs. 86.7%, 100%, 100% for TF). MIBI and TF SMM showed lower sensitivities for the tumors with less than 1 cm than tumors with more than 1 cm. However, the location of tumors did not influence the sensitivity and specificity between MIBI and TF SMM. Conclusion: The ability for the differential diagnosis of breast tumor is similar between MIBI and TF SMM, and delayed image is not necessary. TF may be better than MIBI considering the specificity of SMM without clinical information and labeling convenience.