Water was removed from slurry containing incinerated sludge ash with a spiral cross-flow dewatering system. Fluid resistance of the slurry was measured for different concentrations and pHs of the slurry to evaluate its fluid characteristics. The effect of dispersant addition on fluid resistance and dewatering capability of spiral cross-flow was also evaluated. At solid contents of 25% and above, the slurry showed pseudoplasticity, and with increasing solid content, the fluid resistance increased exponentially. The effect of pH control on fluid resistance was comparably small. However, dispersant addition decreased the fluid resistance, and variations in fluid resistance with shear rate became small. With the spiral cross-flow dewatering system, continuous dewatering of slurry at a high dewatering rate was feasible, and a high dewatering ratio was achievable by adding dispersant.