Crystal structure variation and thermoelectric properties of binary rhenium silicide with different heat treatment conditions are investigated. In quenched rhenium silicide, dense planar defects are observed and the crystal structure is identified as crystallographic shear structure with crystallographic shear operation of ((1) over bar 09)(C11b) / [100](C11b). The crystallographic shear structure observed in quenched samples is not thermally stable. The structure is annealed out by prolonged heat treatment at relatively low temperature. Thermoelectric properties of quenched and annealed rhenium silicide are significantly different. The quenched sample is an n-type semiconductor, while the annealed sample is a p-type semiconductor. Planar defects in the quenched sample are expected to introduce a donor level in the band gap and the electrical conduction becomes n-type.