Li1−3 x Na2 x Mg x Mn0.8Fe0.2P1− x Si x O4 (x =0, 0.01, 0.02, 0.04, 0.08, 0.16) are studied as cathode materials for lithium ion batteries. Na, Mg and Si can be incorporated into the structure of LiMn0.8Fe0.2PO4 without introducing any impurity phase when x is not higher than 0.08, but NaFePO4 appears as an impurity phase when x is as high as 0.16. Charge balance is maintained in Li1−3 x Na2 x Mg x Mn0.8Fe0.2P1− x Si x O4 because the oxidation states of Na, Mg, Mn, Fe, P and Si can be considered as +1, +2, +2, +2, +5 and +4, respectively. The first charge/discharge capacity, the discharge capacity at each current density and the capacity retention upon cycling first increase and then decrease with increasing x, and the maximum values are always obtained when x is equal to 0.01. At the current density of 20 C, Li0.97Na0.02Mg0.01Mn0.8Fe0.2P0.99Si0.01O4 has a discharge capacity as high as 84mAhg−1 and 98.3% of the initial discharge capacity is retained after 100 cycles.