Perovskite-related compounds RTa 3 O 9 (R rare earth elements except Sc, Pm, Yb, Lu) have been prepared and magnetic susceptibility measurements have been carried out. The characteristic behavior in the temperature dependence of the susceptibility for each sample has been analyzed in terms of the crystalline electric field of cubic symmetry yielded from the nearest neighboring 12 O 2ฯช and the next-nearest neighboring eight Ta 5ฯฉ around R 3ฯฉ ions. These analytical results demonstrate clearly that the contribution of the next-nearest neighbors to the susceptibility is important and fairly large because of the high valence state of Ta 5ฯฉ . The ground states of R 3ฯฉ ions in the compounds are determined as follows in the Bethe's notation: Ce 3ฯฉ , G 8 ; Pr 3ฯฉ , G 3 ; Nd 3ฯฉ , G 6 ; Tb 3ฯฉ , G 5 ; Dy 3ฯฉ , G 8 ; Ho 3ฯฉ , G 5 ; Er 3ฯฉ , G 8 ; and Tm 3ฯฉ , G 5 . The precise analysis for RTa 3 O 9 based on the crystal field theory has systematically provided the energy splitting scheme, the f-electron wave functions and significant crystalline field parameters containing both fourth and sixth order terms. The splitting into G 6 , G 7 and G 8 from the degenerated ground state 8 S 7/2 of Gd 3ฯฉ is negligibly small and not distinguishable above 2 K.