High aspect ratio aluminum diboride (AlB₂) single crystal flakes in epoxy have been shown to have an exceedingly high elastic modulus (484 GPa) and an outstanding capability for planar reinforcement. Typically, the flakes are crystallized from an Al-B melt and are formed with aspect ratios of 300/1 measuring several mm in width and with thickness values of 2-10μm. However, their use as a reinforcement in epoxy matrix composites was limited due to the presence of the α-AlB₁₂ cuboidal phase and other related higher borides. High aspect ratio flakes can be prepared from the commercially available material, consisting of low aspect ratio AlB₂ in Al, by heating near the peritectic temperature of 956ºC. The flakes can be further concentrated to 30-35v/o by filtration at 675ºC. Under these conditions we were able to minimize formation of α-AlB₁₂ to well below 1v/o. Composites produced were then examined for flake volume fraction, aspect ratio, orientation, and quantitative degree of flake self-alignment and their effects on the Young’s modulus and strength in flexure. With 30v/o flakes in Al, a modulus of 125GPa was observed with relatively low degrees of alignment of flakes. Improvements in both of these properties will be observed with better alignment of the flakes.