By using the definition of generalized secondorder moments and co m plex Gaussian functions expansion of rectangular function, the analytical expres sions for the generalized M2factor and mode coherence coefficients of th e new m odel of flattopped beams introduced by Li have been derived. The mode correlat i on, mode structure, and coherentmode decomposition of flattopped beams have be en analyzed. Theory of the coherentmode decomposition of truncated flattoppe d beams is built.