Here we report the characteristics of mutual-substituted bi-phase composites with nominal formula of (1 − x)LiMnPO4·xLi3V2(PO4)3/C (x = 0.01, 0.03, 0.05, 0.1, 0.15, 0.2, 0.3, 0.4, 0.5). According to the results of refinement and XPS, as x ≥ 0.03, the prepared material becomes the binary mixture of LiMn1−y V y PO4 and Li3V2−z Mn z (PO4)3 where y is probably ramped rapidly and z is relatively stable. Tested between 2.5–4.4 V vs. Li/Li+, the electrochemical performance of the synthetic composites presents the Plough-shape tendency as the increase of Li3V2(PO4)3's nominal ratio. The discharge capacity is dropped from 99 mAh g−1 of 0.99LiMnPO4·0.01Li3V2(PO4)3/C to 69 mAh g−1 for 0.95LiMnPO4·0.05Li3V2(PO4)3/C, then recuperated, and finally stabilized at ∼125 mAh g−1 for the samples with 20% Li3V2(PO4)3 or more. Compared with LiMnPO4/C, the “gaining point” of composites is incorporating 20% Li3V2(PO4)3, probably corresponding to ∼8% vanadium substitution based on refinement results.