A method and apparatus for quality assurance (QA) of beam geometry of robotic radiosurgery systems. The QA of beam geometry is of paramount importance in achieving clinical precision of robotic radiosurgery treatment, which depends on the accuracy and reproducibility of each beam direction. The method and apparatus of this invention verifies pre-defined beam geometry by radiographic visualization of the beam central axis to better detect inaccuracy of the imaging system and error in robotic precision. The apparatus consists of two radio-opaque markers set at a fixed distance from each other and contained in a housing, which is assembled to a collimator fixture for attachment to the collimator interface of the linear accelerator (LINAC) of the robotic radiosurgery system. A treatment plan is generated to position the LINAC to a series of pre-defined radiation beam orientations, then a simulation of the treatment is carried out with the apparatus attached to the LINAC and images are taken at each LINAC position. The coordinates of the two radio-opaque markers obtained from radiographic images are used to determine beam orientation by calculating their directional cosines, the beam off-axis error and the deviation of the radiation source to iso-center distance (SAD). Once the imaging system is independently calibrated, the quantitative beam geometric information is used to adjust beam geometry to the optimum specification.