We demonstrate the existence of surface waves in two-dimensional photonic crystal slab edges. In finite-sized air-terminated photonic crystal boundaries; the slab-edge mode turns into a lasing mode under pulsed optical pumping conditions. Analyses of the modal behaviors of the slab-edge modes by means of calculations based on the plane wave expansion method and the finite-difference time-domain method show that resonant frequencies and quality factors are strongly dependent on the termination parameter and the shape of the slab corner. (C) 2004 American Institute of Physics.