&#8211; 160 kVp). A full simulation of the photon and electron transport in a kilovoltage x-ray unit, using the Monte Carlo code BEAM/EGS4, was performed to obtain an accurate beam phase-space for use in dose calculation. The x-ray unit was modeled incorporating the primary electron beam, the tungsten target, the Be window, the collimating devices, the additionnal filters, the diaphragm and the monitor chamber. Spectra were calculated from MC simulations for 6 x-ray beams of 10 cm radius at an SSD of 100cm and variable beam quality [measured HVL : 3.36 mm Al – 2.2 mm Cu]. The calculation of HVLs and air kerma show a good agreement with experimental values to within 3.5% and 1.6% respectively. The same model was also used to investigate the change in spectra with target angle. In this part, a modification of only one degree has led to more than 6% difference in HVL for the lowest x-ray energy. This was found to have a minor influence on the ratio of mass-energy absorption coefficients for water to air, (µ en / ρ)w,air and the backscatter factor B, wich are used to convert air kerma to water kerma on the surface of a water phantom, for low energy (up to 150 kV) x-rays. The modeled phase-space gave an accurate characterisation of kilovoltage x-rays beams to be used in calibration and clinical dosimetry either for diagnostic radiology or for radiotherapy.