In order to study the magnetic anisotropy of Yb in R 2 Fe 1 7 (R = rare earth) compounds, the spin reorientation temperature T S R was measured for (Er 1 - x Yb x 2 Fe 1 5 Al 2 (x = 0-0.4) oriented polycrystalline samples. As a result, T S R shifts toward the lower temperature side with x, and the x-dependence of T S R is found to be similar to that for (Er 1 - x Yb x ) 2 Fe 1 5 Al 2 samples where the nonmagnetic Y 3 + is used instead of Yb. From this experimental observation, it is concluded that Yb in R 2 Fe 1 7 compounds is not in the trivalent Yb 3 + state, but in the divalent nonmagnetic Yb 2 + state.