According to the grain size distribution of viscous debris flows in Jiangjia Gully of Yunnan Province of China, four groups of debris flow slurries with different maximum grain sizes (0.25, 1, 2, 5 and 10mm) were prepared. Each group corresponds to a specific density of debris flow. Rheological tests were performed with ball measuring system, and rheological parameters were then determined using the Herschel-Bulkley model to study how these parameters vary as coarser grains were added to a finer-grained slurry. The results show that all slurry samples behave as shear-thinning fluids, whereas they become less shear-thinning and exhibit higher yield stress when coarser grains were added. For slurries with the same maximum grain size,yield stress increases exponentially with volumetric solid concentration. There is no distinct relationship between the consistency index and the maximum grain size because of the variation in fluidity index. However, the apparent viscosity increases when coarser grains were added, and the increasing amplitude is a little less than that of the yield stress. A good linear relationship exists between the relative viscosity and the relative yield stress of coarser-grained slurries to fine-grained slurries, which can be used to estimate the apparent viscosity of debris flow.