Room-temperature solid-state maser is highly desirable,but lifetimes of emitters(electron spins) in solids at room temperature are usually too short(～ns) for population inversion.Nitrogen-vacancy(NV) centres in diamond,which feature the longest known solid-state spin lifetime(～5 ms) at room temperature,high optical pumping efficiency(～106/sec),and material stability,are promising as a gain medium for room-temperature solid-state maser.