It is well known that fine gangue particles play an important role in contaminating the concentrate during coal flotation process. However, the contamination behavior of coarse gangue particles in the concentrate has been neglected in the past. In this paper, the contamination behavior of coarse gangue particles in the concentrate during coal flotation process was investigated using a mixture of coarse gangue particles and low ash coal particles at a proportion of 1:1. The size fraction of coarse gangue particles was 0.5-0.25 mm. The size fraction of low ash coal particles was 0.5-0.25, 0.25-0.074, and -0.074 mm, respectively. The results of XRD showed that coarse gangue particles were composed of quartz, pearl clay, kaolin, mica, and dickite. The results of contact angle measurements showed that the contact angle of coal particles was about 89 degrees while that of gangue particles was only 30 degrees. The results of flotation tests indicated that the contamination behavior of coarse gangue particles in the concentrate during the coal flotation process cannot be neglected.