Objective: To compare the dosimetric differences in forward intensity-modulated radiotherapy using field-in-field (FIF-F-IMRT) and inverse intensity-modulated radiotherapy (I-IMRT) for the left-sided breast cancer. Methods: A total of 18 patients with left-sided breast cancer undergoing breast-conserving surgery in Shanxi Provincial People's Hospital from January to July 2015 were enrolled. For each patient, two treatment plans were designed. The plans were compared by means of target dose distribution and dose for organ at risk. Results: The two methods met the requirements of the prescribed doses. There were no differences for maximal dose (D_max), mean dose (D_mean), and 100%, 105%, 110% prescription dose percent volume (V₁₀₀, V₁₀₅, V₁₁₀) of the target (all P > 0.05). The percentage volume of heart receiving 5 Gy (V₅), 20 Gy (V₂₀), 30 Gy (V₃₀), D_mean of heart of FIF-F-IMRT [(11±4)%, (7±4)%, (8±3)%, (15±3) Gy] were lower than those of I-IMRT [(42±8)%, (14±10)%, (14±4)%, (18±7) Gy] (t values were 29.457, 5.542, 5.064, 4.165, all P = 0.000). V₅ of left-sided lung of FIF-F-IMRT [(26±6)%] was lower than that of I-IMRT [(47±12)%] (t = 6.708, P = 0.000), but there was no significant difference between the two groups in V₂₀, V₃₀, V₄₀, D_mean of left-sided lung (all P≥0.05). V₅ of right-sided lung and right-sided breast, D_mean of right-sided lung of FIF-F-IMRT [(7±4)%, (26±5)%, (60±19) Gy] were lower than those of I-IMRT [(31±9)%, (48±11)%, (489±67) Gy] (t values were 12.304, 6.708, 42.489, all P = 0.000). The number of the monitor units and spending time of FIF-F-IMRT were lower than those of I-IMRT (t values were 12.214 and 29.899, both P = 0.000). Conclusion FIF-F-IMRT has a good dose distribution in the target volume, and could reduce the dose for organ at risk, especially reduce the unnecessary low dose irradiation, and consume less planning time and equipment consumption.