In 201Tl myocardial perfusion single-photon emission computed tomography (SPECT), gastric wall uptake can impact the inferior wall. This study aimed to evaluate the effectiveness and usefulness of the masking on un-smoothed image (MUS) method for 201Tl myocardial perfusion SPECT. A hemispherical gastric wall phantom was created to simulate the gastric fundus located closest to the myocardium, and the activity was enclosed to achieve an SPECT count ratio against the myocardium equivalent to that observed in clinical practice. The minimum values of the defect chip in the circumferential profile curve were compared for six SPECT count ratios and seven gap distances. In the conventional method, increasing SPECT count ratios or gap distances interfered with myocardial perfusion SPECT evaluation. Artifacts were less apparent when the MUS method was applied. The MUS method effectively suppressed the gastric wall uptake on 201Tl myocardial perfusion SPECT.