This study investigates the flowability of the bulk-molding compound (BMC) on composite bipolar plates containing graphite content from 70 to 80 wt% with different graphite sizes. A small quantity (from 0.25 to 2 wt%) of multi-walled carbon nanotubes (MWCNT) is also added. Findings show that the flowability of BMC material decreases with decreasing graphite size and with increasing graphite content. The BMC material containing large size graphite (177-125 mu m) entirely exhibits a relatively higher flowability within the analysis graphite contents, in the range of 73.3-11.3 cm, compared to the small size (74-45 mu m), in which flowability is in the range of 40.3-6.67 cm. Further adding MWCNT causes decreased flowability of the BMC material especially when the percolated networking structure is formed through the resin. Therefore, with flowability below 10 cm, the formability of a large area (300 mm x 300 mm x 3 mm) or thin (100 mm x 100 mm x 0.5 mm) composite bipolar plate shows a large area of surface porosity or visible defects. Results indicate that the flowability of the thermoset-based BMC material is an important design parameter to fabricate cost-effective, large, or thin composite bipolar plates. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.