A systematic density functional theory study has been carried out on the structure and stability of triangular molybdenum sulfide (MoSx) models. On the basis of the structural and energetic comparison, the triangle Mo28S84 (VII) cluster has been identified as a reasonable structure for triangular MoSx model. Under reductive atmosphere, the most stable structure has bridging sulfur on edge sites and two H2S at each corner site. It is found that CO adsorption at the corner site represents the most stable conformation. Along with other stretching modes, the computed frequency at 2102 cm(-1) for CO at the corner agrees perfectly with the experimental observation.