Room-temperature plasmonic-crystal lasers have been demonstrated with a square-lattice gold nano-pillar arrays on top of InGaAs/GaAs quamtum wells on a GaAs substrate. The lasing wavelength is tunable in the range of 865-1001 nm by varying the lattice period. The lasers exhibit an extremely narrow linewidth and small divergence angle so could have great potential for various applications. An unexpected mirror cavity effect has been observed and investigated. The mirror-cavity lasers have a very low threshold and could be developed to realize electrically-driven plasmonic lasers.