BACKGROUND Megavoltage beams used in radiotherapy are contaminated with secondary electrons. Different parts of linac head and air above patient act as a source of this contamination. This contamination can increase damage to skin and subcutaneous tissue during radiotherapy. Monte Carlo simulation is an accurate method for dose calculation in medical dosimetry and has an important role in optimization of linac head materials. The aim of this study was to calculate electron contamination of Varian linac. METHODS The 6MV photon beam of Varian (2100 C/D) linac was simulated by Monte Carlo code, MCNPX, based on its company's instructions. The validation was done by comparing the calculated depth dose and profiles of simulation with dosimetry measurements in a water phantom (error less than 2%). The Percentage Depth Dose (PDDs), profiles and contamination electron energy spectrum were calculated for different therapeutic field sizes (5×5 to 40×40 cm(2)) for both linacs. RESULTS The dose of electron contamination was observed to rise with increase in field size. The contribution of the secondary contamination electrons on the surface dose was 6% for 5×5 cm(2) to 27% for 40×40 cm(2), respectively. CONCLUSIONS Based on the results, the effect of electron contamination on patient surface dose cannot be ignored, so the knowledge of the electron contamination is important in clinical dosimetry. It must be calculated for each machine and considered in Treatment Planning Systems.