Chimney-ladder compounds with the general chemical formula of Mn X2n-m (n, m: integers) possess tetragonal crystal structures which consist of two types of subcells; one composed of transition metal atoms (M) with the γÀ-Sn structure and the other composed of group 13 or 14 atoms (X) with a helical arrangement along the tetragonal c-axis. Since the chimney-ladder compounds generally exhibit very low thermal conductivity, presumably due to its long periodicity along the c-axis, they have been extensively investigated as promising thermoelectric materials. The high-temperature (HT) phase of Ru2Si3 is one of the chimney-ladder compounds with n=2 and m=1. Recently we have found that the HT-Ru2Si3 phase is stabilized by substituting Ru with Re so as to exist even at low temperatures in a wide compositional range of the Re content (Re: 14 to 73%), and that the thermoelectric power factor for alloys with high Re contents increases with the Re content and the highest value was obtained for the alloy with the highest Re content (73%), which is the solubility limit of Re in the chimney-ladder phase. In order to further enhance the thermoelectric properties, another ternary element which extends the solid solubility region of the HT-Ru2Si3 phase is favorable. We have chosen Mn as the ternary element because Mn4Si7 with the chimney-ladder structure exists as a counterpart of HT-Ru2Si3 in the Ru2Si3 -Mn4Si7 pseudo-binary system so that the solid solubility region of the chimney-ladder phase is anticipated to extend in a wider composition range than the Re case. Our study, in fact, shows that the Mn-substitution stabilizes the HT-Ru2Si3 phase in a wide compositional range of the Mn content; 12 to 100%. Compositional analyses indicate that the Si/M ratio gradually increases as the Mn content increases. This is considered to be due to the addition of Si atoms in the Si subcell in order to compensate the decrease in the valence electron concentrations (VEC) per M atom by the substitution of Ru (group 8) with Mn (group 7) with fewer valence electrons. The Seebeck coefficient and electrical resistivity of the Mn-substituted Ru2Si3 are explained in terms of the VEC deviation from the idealized value, 14, which is expected for intrinsic semiconductors with the chimney-ladder structure. The highest dimensionless thermoelectric figure of merit (ZT=0.76) is obtained for 90%Mn-substituted alloy. The relationships between the microstructure and thermoelectric properties will be discussed.