Fly ash microsphere (FAM) is a superfine fly ash. Being finer than cement, it can fill into the voids between cement grains. To study the effects of adding FAM, cement paste samples with FAM contents ranging from 0 to 60% and different water contents were made for packing density, flowability and strength measurements. The results showed that the addition of up to 40% FAM would significantly increase the packing density of the cementitious materials. With the voids partially filled by FAM and the same volume of water in the voids released, the addition of FAM could increase the water film thickness of the cement paste. Consequently, the addition of FAM could increase flowability at same strength, increase strength at same flowability, or increase both strength and flowability. Hence, FAM is an effective cementitious filler for improving the performance of cement paste and concrete. (C) 2012 Elsevier B.V. All rights reserved.