report optically pumped room-temperature lasing in a square-lattice photonic crystal cavity consisting of two shifted lattice points and heterogeneous lattices with different air hole sizes. Three-dimensional finite-difference time-domain simulation shows that this cavity has an ultrasmall mode volume of 0.017 mu m(3) similar to 1.7 (lambda/2n(slab))(3). Photoluminescence spectroscopy exhibits pulsed lasing action at 1511 nm with a low lasing threshold of similar to 130 mu W. The measured Q factor is similar to 2400 that is well agreed with the theoretical Q of 4200. In addition, the computational analysis based on the actual fabricated structural parameters unambiguously presents the successful demonstration of this ultrasmall lasing mode. (c) 2008 American Institute of Physics.