Though adjuvant radiotherapy has an established role in the treatment of carcinoma breast, there is concern regarding irradiation to heart and lung, more on left sided disease. Intensity Modulated Radiation Therapy (IMRT) modulates the intensity of the radiation beams with better accuracy, sparing surrounding normal organs. But it increases integral dose to normal healthy tissues compared to 3-Dimensional Conformal Radiation Therapy (3DCRT). So, our study was aimed at comparing the dosimetry and acute toxicity profile of 3DCRT and IMRT in post-mastectomy patients. In study arm patients received radiotherapy with IMRT and in control arm with 3DCRT, radiation dose being 50 Gy in 25 fractions for 5 weeks for both the arms. We compared the dosimetric data for Planning Target Volume (PTV) and Organ At-risk (OAR) by both techniques along with acute toxicity profile. Dosimetric parameter of PTV coverage V95 and V107 were significantly better in IMRT than 3DCRT (p-value <0.0001). IMRT showed better homogeneity index (0.14 Gy vs 0.26 Gy) and conformity index was also better for IMRT (0.94 Gy vs 0.74 Gy, p-value 0.0028). The mean value of heart V25 in left sided disease was significantly lower in IMRT than 3DCRT (22.59 and 25.64, p value 0.01). V20 of ipsilateral lung was numerically less in IMRT though not significant (31.44 vs 35.3). But low dose volume was significantly more in IMRT, as seen by higher V5 of heart and ipsilateral lung. To conclude, IMRT has better PTV coverage and better sparing of organs at risk with more homogenous and conformal plans than 3DCRT.