The orthorhombic compound Ru2Si3 is currently of interest as a high-temperature thermoelectric material. In order to clarify the effects of crystal orientation on the thermoelectric properties of Ru2Si3, we have examined the microstructure, Seebeck coefficient, electrical resistivity, and thermal conductivity of Ru2Si3 along the three principal axes, using these measured quantities to describe the relative thermoelectric performance as a property of crystal orientation. Ru2Si3 undergoes a high temperature (HT)→low temperature (LT) phase change and polycrystalline Si platelet precipitation during cooling, both of which are expected to effect the thermoelectric properties. The HT tetragonal→LT orthorhombic phase transformation results in a //, // two-domain structure, while polycrystalline Si precipitation occurs on the (100)
planes. The  orientation is found to posses superior thermoelectric properties (with the dimensionless figure of merit, ZT/ZT>4 at 650°C), due principally to the larger Seebeck coefficient along the  direction. The effect of the domain structure on the thermoelectric properties is discussed.