We have investigated phase relationships of the sesquisilicide alloys in the Ru-Mn-Si system. A series of chimney-ladder phases Ru(1-x) Mn (x) Si (y) (0.14 a parts per thousand currency sign x a parts per thousand currency sign 0.97, 1.584 a parts per thousand currency sign y a parts per thousand currency sign 1.741) are formed over a wide compositional range between Ru(2)Si(3) and Mn(4)Si(7). We also investigated thermoelectric properties of the directionally solidified Ru(1-x) Mn (x) Si (y) alloys as a function of Mn content and temperature. The dimensionless figure of merit ZT for alloys with high Mn content (x a parts per thousand yen 0.75) increases as the Mn content increases. The alloy with x = 0.90 exhibits ZT as high as 0.76 at 874 K.