The influence of atmospheric turbulence on partially coherent flat-topped Gaussian beam is investigated in this paper. Partially coherent flat-topped Gaussian beam is expanded as a superposition of independent Hermite-Gaussian modes. Propagation of laser beam through the turubulent atmosphere is numerically studied using phase screen approximation. We deduc two expressions to fit the results of numerical simulation. One describes the dependence of the rms width of the beam on the order number N, the psreading distance and the intensity of turbulence. The other describes the dependence of the coherent length of the beam on that of its initial time and that of turbulence.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.