Purpose To evaluate the effect of multileaf collimator (MLC) leaf width on dosimetric parameters and delivery efficiency of volumetric-modulated arc therapy (VMAT) for breast cancer after conservative surgery.Methods and materials Nineteen patients with T 1 NO carcinoma (12 left-sided and 7 right-sided) treated with conventional technique in our clinic were selected for this retrospective analysis.VMAT treatment plans with each of three kinds of Elekta MLCs (Beam ModulatorTM (BMTM) with 4-mm leaf width, AgilityTM with 5-mm leaf width and MLCi2 with 10-mm leaf width) were generated independently for each patient.Plans comparison were done based on dose-volume histograms (DVH) analysis including planning treatment volume (PTV) dosimetric parameters such as homogeneity index (HI), conformity index (CI), Dmax, Dmin and Dmean as well as organ at risks (OARs) dose and volume parameters.Delivery efficiency of the three types of MLCs was compared in terms of beam delivery times and monitor units (MUs) per fraction for each plan.