The fluidity of Coal Water Mixture (CMW) is significantly lost when CWM is heated over 100℃. This Phenomenon has been analyzed experimentally in this work. Experimental results showed that the degradation of additives at higher temperature range and the extraction of organic substances from coal to solvent were not the reason for viscosity increase of CWM. It was also observed that there existed a little swelling of coal perticles at 150℃～200℃. Semiquantitative discussion about the effect of swelling on the fluidity of CWM has revealed that the swelling of coal is most likely the reason for the anomarous viscosity increase of CWM at higher temperature range.