The rheological properties of a highly loaed coal water mixture (CWM) were investigated according to the CWM Quality TEST Method (CCUJ United Method). The representative apparent viscosity and viscosity power index were both increased with weathering of the feed coal. It was considered that acidification and an increase in hydrophilic functional groups on the coal surface were the main causes of the changes. The representative apparent viscosity increased and viscosity power index decreased with the increase in ash contents of feed coal. It was considered that clay minerals liberated from the coal surface caused the changes. Agitation was found to be very effective to decrease the representative apparent viscosity of a CWM and to improve rheological properties. The rheological properties greatly changed with coal concentratin and temperature of the CWM, while they changed only slightly with the amount of dispersant added and with weight percent of particles over 75 mu m in size.