In order to evaluate the task specific measurement uncertainty of laser tracker, based on the analysis of the uncertainties of the laser tracker system(LTS) ,the Monte-Carlo(MC) method is proposed and applied, and it's principle and steps are described. We have measured the flatness of the marble table with a laser tracker and a CMM, and compared with the MC uncertainty simulation. The results of the MC method and laser tracker measurements are in well agreement with each other, so we conclude that the MC methods could be used in the evaluation of the task-specific measurement uncertainty of laser tracker.