The dependence of thermoelectrical efficiency Z of thermoelements based on Bi_0,5Sb_1,5Te₃ and Bi₂Te_2,7Se_0,3 on the branches materials resistivity r at different values or resistances or transitional contacts r and branches length l; on branches length l at different r and r; and on transitional contacts resistance r at different r and l of branches have been studied. It have been found, that for given commutation alloy the length l_k, after which the strong reduction of D T max had been observed with decreasing branches length, does not depend on branches resistivity. Although, at the transition from alloy with small contact resistance to the large ones, l_k is shifted to large branch lengths. It can be explained by the fact that for the same alloy r itself depends on crystals r ; the majority charge carriers concentration in crystal is decreased with increasing r , that leads to the increase of crystal - contact alloy Junction resistance. It have been round that r of contacts is directly proportional to branches r . There fore, the ratio r/r remains r is changed under the transition from one alloy to the other, and it leads to the shifting of l_k value.