Background: Gold nanoparticles are emerging as promising agents for cancer therapy and are being investigated as drug carriers, photothermal agents, contrast agents and radiosensitisers.Objective: The aim of this study is to understand characteristics of secondary electrons generated from interaction of gold nanoparticles GNPsÂ with x-rays as a function of nanoparticle size and beam energy and thereby further understanding of GNP-enhanced radiotherapy. Methods: Effective range, defection angle, dose deposition, energy, and interaction processes of electrons produced from the interaction of x-rays with a GNP were calculated by Monte Carlo simulations. The MCNPX code was used to simulate and track electrons generated from 30 and 50 nm diameter GNP when it is irradiated with a cobalt-60 and 6MV photon and electron beam in water. Results: When a GNP was present, depending on beam types used, secondary electron production increased by 10- to 2000-fold compared to absence of a GNP. Conclusion: GNPs with larger diameters also contributed to more doses.