The direct synthesis of metastable olivine phase NaMnPO4 still remains a challenge. In this study, single crystals of the olivine NaMnPO4 were synthesized for the first time by a facile hydrothermal reaction. Single-crystal X-ray diffraction analysis provides precise structure information on the olivine structure of NaMnPO4. Moreover, the olivine NaMnPO4 can be completely transformed to its maricite phase after heating at 600 ​°C under Ar atmosphere, confirming the metastable nature of the olivine phase. Electrochemical performance of the olivine NaMnPO4 as anode material for Li-ion batteries (LIBs) was characterized. It delivers a capacity of 645.0 mAh g−1 ​at 0.1 ​A ​g−1 after 200 cycles. In contrast, the maricite phase NaMnPO4 exhibits very low lithium storage capacity. Single crystals of the metastable olivine phase NaMnPO4 were synthesized for the first time which exhibits much higher capacity as anode material of Li-ion batteries than that of the maricite phase. Display omitted •Single crystals of the metastable olivine phase NaMnPO4 were synthesized for the first time.•The olivine to maricite phase transformation of NaMnPO4 was verified.•The olivine phase NaMnPO4 exhibits much higher capacity than maricite phase as anode material of Li-ion battery.