Phase diagrams play a very important role in alloy design to forecast possible microstructures that would be controlled by alloying process and heat treatment. A good example, to see how phase diagrams are practically used, is our alloy design on E2₁ Co₃AlC based heat resistant alloys. We have already reported that Co₃AlC based multi-phase alloys are attractive candidates since they have a good balance of elevated temperature strength and ambient temperature ductility. To further improve their mechanical properties, microstructure control has been conducted using optical floating zone melting, which includes growing single crystal and controlling eutectic lamellar microstructure. It is necessary to evaluate and to consider several pieces of information of the Co-Al-C ternary phase diagram, particularly of phase equilibria involving Co₃AlC and liquid phase. We have succeeded in growing Co₃AlC single crystals for the first time, which would never have been achieved without utilizing the phase diagram information.