We investigate optical characteristics of the ultra-small zero-cell cavities that consist of two, four and three shifted lattice
points in square- and triangular-lattice photonic crystal structures. Mode volumes and Q factors of these cavities are
systematically studied using three-dimensional finite-difference-time-domain simulation. In particular, an extremely
small mode volume of ~0.015 μm<sup>3</sup> [~1.5 (λ/2<i>n</i><sub>slab</sub>)<sup>3</sup>] is obtained in the triangular-lattice three-hole-shifted cavity. In an
experiment, we demonstrate optically pumped room-temperature lasing action with a low lasing threshold of ~130 μW in
a square-lattice two-hole-shifted cavity. The operation of this ultra-small laser is unambiguously confirmed by the
numerical simulation based on the actual fabricated structures.