We evaluated the effect of relative phase-shift of cardiac and respiratory (C&R) motions in myocardial perfusion (MP) ECT. Previously, we generated a set of realistic 3D XCAT (eXtended CArdiac Torso) phantoms that model simultaneous C&R motions for use in the study of new data acquisition methods and corrective image reconstruction techniques for improved gated MP ECT, including PET and SPECT. The respiratory motion (RM) over a respiration cycle was modeled using 24 equally-spaced time frames while the cardiac beating motion (CBM) over a cardiac cycle was divided into 48 equally-spaced time frames for each of the 24 RM phases. Almost noise-free projection datasets were generated separately from the heart, blood pool, lungs, liver, kidneys, stomach, gall bladder and remaining body at each of the 24x48 time points using Monte Carlo simulation techniques that include the effect of collimator detector response, photon attenuation and scatter. To demonstrate the effect of relative phase-shift, a typical 99mTc Sestamibi MP SPECT projection dataset were generated. They were then scaled and combined to model different degrees of relative C&R phase shifting and grouped into 6 respiratory-gates with 8 cardiac-gates. Each projection was reconstructed using a 3D OS-EM without and with attenuation correction using an averaged and phase-mismatched gated attenuation maps. The image artifacts of the reconstructed images were compared by visual inspection of the MP polar maps. The results showed significant changes of artifactual non-uniformity in the polar maps for off-phase of RM compared to those of CBM. The changes in the polar maps also demonstrated the effect of phase shifting accordingly. We conclude that the 4D XCAT phantom dataset with simultaneous C&R motions provides a powerful tool in the study of the effects of C&R motions with relative phase shifts, and development of C&R gating schemes and motion correction methods for improved ECT/CT imaging.