Single-crystal cavities of p-sexiphenyl (p-6P) are prepared by precipitation from a solution phase. A typical p-6P crystal has a thin quasi-lozenge shape with a size of several hundreds of um and a thickness of a few um. Under optical pumping with a pulsed laser, blue fluorescence is gain-narrowed to amplified spontaneous emission (ASE) due to light confinement inside the slab-type crystal cavity. With increasing excitation fluence, the time profiles of photoluminescence change from an exponential decay of spontaneous regime to a pulsed one typical for stimulated emission. Furthermore, multimode laser oscillations are obtained when a pair of parallel facets of the crystal cavity function as feedback mirrors.