This study aimed to estimate the radiation-induced second cancer risks in normal tissues after the treatment of breast cancer with three-dimensional conformal radiotherapy (3DCRT) and intensity-modulated radiotherapy (IMRT). In this study, 10 patients with breast cancer have been chosen.Three-dimensional conformal radiotherapy (3DCRT), intensity-modulated radiotherapy (IMRT) plans were designed. The 3DCRT and IMRT plans were evaluated and compared with several dosimetric parameters for Planning Target Volume (PTV) and the Organs at Risk (OARs). The Organ Equivalent Dose (OED) was calculated based on linear, linear-exponential, and plateau dose-response models. The Second cancer risks were estimated by Excess absolute risk (EAR) for OARs. For breast, PTV dose coverage parameters were significantly improved in IMRT compared to 3DCRT.The ipsilateral lung V30Gy % and V20Gy % were significantly decreased with IMRT. However, The IMR plans were shown significant increases (p < 0.05) in the mean doses Dmean to OARs. Compared with the 3DCRT, the IMRT plans increase OED in OARs based on the linear, plateau and linear-exponential models. The second cancer risk with IMRT increased by 2.5 -3.5fold, 3.3-4fold, 3-7fold, and 4.6-5.3fold for contralateral lung, contralateral breast, esophagus, and stomach based on dose-response model applied. Conclusion: IMRT technique demonstrated a clear advantage in dose coverage, conformity, and homogeneity over 3DCRT and was superior in terms of OAR-sparing. The Second cancer risk in normal tissues based on the EAR model after IMRT is higher than 3DCRT. Advanced radiotherapy techniques as IMRT for breast cancer treatment must be evaluated based on secondary cancer risks when treating young patients