The application of masers is limited by its demanding working conditions (high vacuum or low temperature). A room-temperature solid-state maser is highly desirable, but the lifetimes of emitters (electron spins) in solids at room temperature are usually too short (～ns) for population inversion. Masing from pentacene spins in p-terphenyl crystals, which have a long spin lifetime (～0.1?ms), has been demonstrated. This maser, however, operates only in the pulsed mode. Here we propose a room-temperature maser based on nitrogen-vacancy centres in diamond, which features the longest known solid-state spin lifetime (～5?ms) at room temperature, high optical pumping efficiency (～106?s?1) and material stability. Our numerical simulation demonstrates that a maser with a coherence time of approximately minutes is feasible under readily accessible conditions (cavity Q-factor ～5 × 104, diamond size ～3 × 3 × 0.5?mm3 and pump power <10?W). A room-temperature diamond maser may facilitate a broad range of microwave technologies.
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