To investigate fluidity and hydrodynamic behavior of investment slurry, a fluidity testing apparatus was machined and constructed.The apparatus was consisted with the reservoir for investment slurry, 4 nozzles (0.60, 0.85, 1.05 and 1.20cm diameter with 4.00cm length) and the receptacle withen two electrodes for detecting the volume flow rate of the slurry. The flow rate of the slurry of the cristobalite investment were measured every 20ml to 100ml using 4 nozzles on each W/P ratio (0.33, 0.31, 0.29) . The average velocity was calculated from an average flow rate between 0 to 60ml. Flow rate measurement revealed the relatonship between the average velocity(v)of the slurry and diameter (d) of the nozzle used approximately by the Poiseuille equation in each W/P tested.This means that the flow of the investment slurry behave likely the Newtonian. The value of 32・l・v/d²(l=length of nozzle)indicated the fluidity of the investment slurry.Therefore, the fluidity of some investments can be evaluated by comparison with calculated values of 32・l・v/d² from obtained average velocity, diameters and length of nozzles used.