A double perovskite-structure oxide La2LiFeO6 with unusually high-valence Fe(5+) was synthesized using a high-pressure technique. The Li(+) and Fe(5+) ions at the B site in the rhombohedral R3̅ perovskite structure are ordered in a rock salt manner, and the resultant tetrahedral network of Fe(5+) gives geometrical spin frustration, which is consistent with a large frustration index f (|θ|/TN) ≈ 10. Mg(2+) substitution for Li(+) produces Fe(4+) from some Fe(5+) and changes the magnetic properties. The Weiss temperature is increased from -119 to 21 K by the substitution of only 1%, significantly decreasing the frustration index. The geometrical frustration of the Fe(5+) spin sublattice cannot be tolerant for even a very small amount of Fe(4+) disturbance.