A partially Fe-substituted solid solution of a higher manganese silicide, (Mn1-xFex )Siγ , has been prepared by means of arc-melting and a subsequent annealing process. According to the linear relationship between x and lattice parameters, the solid solution can be prepared up to x = 0:35. The compounds consist of two tetragonal subsystems of [Mn1-xFex ] and [Si], with an irrational c-axis ratio γ = cMn=cSi ̃ 1:7. With increasing x from 0 to 0.35, equivalent to increase electron carriers, the a-axis and cMn-axis lengths gradually decrease, while the cSi-axis length linearly increases. On the basis of a valence electron counting rule, we expected that a transition from p-type to n-type would occur at x = 0:23 ± 0:05. This transition is experimentally confirmed at x ̃ 0:28; the n-type samples are prepared at 0:28 ≤ x ≤ 0:35. The electrical conductivity at room temperature exhibits symmetrical curves versus x for both p- and n-type samples. However, the solubility limit of Fe appears to suppress any further increase in carrier concentration, resulting in poor electrical conductivity for n-type samples. As a consequence, the maximum dimensionless figure-of-merit ZT = 0:071 at 644 K is obtained for a sample with x = 0:30.