Porous　single　crystal　has　the　characteristics　of　long-range　order,　continuous　lattice　and　large　specific　surface　area,　which　could　reduce　energy　losses　and　keep　high　activity　and　stability　in　electrochemical　systems.　Here,　we　grow　porous　single-crystalline　and　polycrystalline　molybdenum　nitrides　microcubes　from　MoO3　single　crystals.　These　porous　microcubes　show　superior　HER　and　OER　performance.　The　overpotential　of　Mo2N　porous　single　crystal　microcubes　is　only　73.13　mV　at　a　current　density　of　10　mA　cm−2,　which　is　150.53　mV,　192.76　mV　and　255.87　mV　lower　than　that　of　MoN　single　crystal,　Mo2N　polycrystal　and　MoN　polycrystal,　respectively.　The　advantages　of　Mo2N　porous　single　crystals　in　electrocatalytic　properties　are　also　reflected　in　OER.　©　2021　Hydrogen　Energy　Publications　LLC