Background: In the radiotherapy of patients with esophagus and breast cancer, the heart receives a significant dose of radiation that might cause heart complications. Thefore, the aim of the current study was to evaluate the effects of esophagus and left breast cancer radiotherapy on the cardiac function and to determine the relationship between the dosimetric parameters and ejection fraction (EF) changes. Materials and Methods: Patients with esophageal (n=13) and left breast cancer (n=21) enrolled at our radiotherapy center from March to October 2017. Echocardiography tests were obtained from patients, before and six months after the radiotherapy. Dosimetric parameters were extracted from treatment planning system. The assessed outcomes included pre- and post-radiation EF ratios and percentage change in the EF following radiation.  Results: The mean ± standard deviations of EFs in patients with breast cancer before and six months after treatment were 55.95%±3.2% and 53.10%±6.30% respectively, which were not statistically significant (P = 0.07). In patients with esophagus cancer, the mean ± standard deviations of pretreatment and post-treatment EFs were 56.76%±3.44% and 52.09%±3.88% respectively, which were statistically significant (P = 0.005). Conclusion: The results of our study showed a significant variation of EFs in esophageal cancer patients following radiotherapy, while breast cancer patients treated with radiotherapy showed no significant change. In patients with esophagus cancer, there was a significant correlation between the variation of EFs and volume of heart receiving radiation doses ≥30 Gy (≥V30). Therefore,  to avoid reduction in EF, the use of V20 as a dose-volume constraint is recommended.