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 -μ3 [∼1.5 (λ/2n slab)3] 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 -W 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.