Manganese　oxide　nanocrystals　are　of　great　interest　for　producing　advanced　high-　performance　lithium　ion　batteries　owing　to　the　shortened　lithium　ion　diffusion　length　and　accelerated　interfacial　charge　transfer　rate.　Here　we　have　developed　a　well-　controlled　generic　method　to　synthesize　monodisperse　MnO　nanocrystals,　and　present　a　comparative　study　regarding　the　effect　of　crystallite　size　on　electrochemical　stability.　Nanocrystalline　MnO　with　a　size　of　about　10　nm　shows　the　optimal　lithium-　storage　performance.　Notably,　Mnbased　nanocrystals　retain　their　stable　cyclability　and　excellent　highrate　performance　as　both　the　anode　and　cathode.　The　allnanocrystal　MnO/　C//　LMO　Li-　ion　full　cells　not　only　significantly　improve　the　electrochemical　properties　of　Mn-　based　materials　but　also　open　up　avenues　for　the　future　development　of　various　energy　devices.