The crystal structure of the defect disilicide formed with Re (ReSi1.75) has been refined by transmission electron microscopy combined with first-principles calculation. The crystal structure is monoclinic with the space group Cm (mc44) due to an ordered arrangement of vacancies on Si sites in the underlying (parent) C11(b) lattice. The thermoelectric properties of ReSi1.75 are highly anisotropic. Its electrical conduction is of n-type when measure along [001] while it is of p-type when measured along [100]. Although the value of Seebeck coefficient along [100] is moderately high (150-200 muV/K), it is very high along [001] (250-300 muV/K). As a result, a very high value of dimensionless figure of merit (ZT) of 0.7 is achieved at 1073 K when measured along [001]. Upon alloying ReSi1.75 with Mo, an incommensurate microstructure, in which two block layers with different chemical compositions and vacancy concentrations stack randomly along [100] in the parent C11(b) lattice, appears. The ZT value is further increased to 0.8 with a small amount (2% substitution for Re) of Mo addition.