Thermoelectric properties of Fe3Si with the DO3 structure where some Fe were replaced with Mn and V were examined. By adding 5 mol% Mn or V, the Seebeck coefficients, alpha, reach the maximum values of about +22.5 micro V/K, while they decreased to a large negative value by the further replacement of Fe by these transition elements. The minimum values of alpha were -36 micro V/K and -45 micro V/K at Fe-30 mol%Mn-25 mol%Si and Fe-20 mol%V-25 mol%Si, respectively. These values are smaller than the minimum value reported for the iron-based alloys. The compositional dependency of alpha corresponds to the fact that the added transition metal atoms replaced iron atoms at a particular site in the DO3 lattice. By combining an approximate model approximation suitable for simple metals and the density of state reported, however, the obtained compositional dependency of thermoelectricity could not be explained. The absolute values of alpha in the Fe-Mn-Si system were small, both in the samples with the alpha-Fe solid solution and with the gamma-Fe solid solution.