Abstract: Partially coherent Gaussian beams which can be expanded into an independent superposition of Hermite-Gaussian modes have been analyzed. The analytical expressions for the mode coefficients, the weighting factors and the M2-factor of partially coherent Gaussian beams have been derived. Relation of weighting factors and M2-factor has been discussed. It can be shown from the research work that the mode coefficients, the M2-factor and the mode structure can be determined by means of intensity detection.