The effect of powder properties of filler on the flowability for molding compound of thermosetting plastic was investigated. The size distribution of filler was varied with classifiers. The compressive property and tapping property of fillers were measured by a compressive strength tester and a powder property tester. The flowability of filler in the form of slit flow and spiral flow was compared. Experimental results showed that 1) the powder packing characteristics such as the compressive property or tapping property affect the flowability when the particle size is considerably smaller than the channel thickness of a die, 2) the particle size distribution, especially the volume fraction of coarse particles, directly affects the flowability of filler when the particle size is close to the channel thickness.