To examine crosstalks (CRs) between 99mTc-methoxyisobutyl isonitrile (MIBI) and beta-methyl-p-(123I)iodophenyl-pentadecanoic acid (BMIPP), myocardial phantom studies of simultaneous acquisition were performed with three window widths and four energies where the window located. CRS in 99mTc window were less than 10.5% regardless of the window settings. CRs in 123I window were ranging from 17.0% to 8.6% depending on the window setting. There were significant correlations between the isotope concentration and counts of 99mTc and 123I, respectively (r = 0.98, p < 0.0001). We concluded that the optimal window setting of 99mTc-MIBI and 123I-BMIPP (10% width, 140 keV center 99mTc window and 10% width, 164 keV center 123I window) yields good quality images in dual isotope SPECT.