While preparing parallel mirrors for edge-emitting organic semiconductor lasers (OSLs) by cleaving, the edge of an organic layer is severely damaged typically by the force of cleaving the substrate. By using an organic layer hardened at a low temperature, we were able to cleave an organic layer along the facet of the substrate and reproducibly obtain smooth and parallel mirror surfaces. Slab waveguide OSL structures consisting of 200 nm-thick Alq(3):DCM films (5% DCM) were vacuum-deposited onto polished GaAs (100) substrates coated with an 800 nm-thick layer of RF-sputtered SiO2. The edge facets were prepared by cleaving the OSL structures in liquid nitrogen, and the facets of the organic layers were evaluated by scanning electron microscope. The samples were optically pumped using a nitrogen laser (lambda=337 nm) with 600 ps pulse width at a 20 Hz repetition rate. The typical threshold power density was 68 mu J/cm(2) in the sample with about a 10 min cavity length. The lasing peak wavelength was 644.5 nm. The full width at half maximum of the photoluminescence spectrum, dependence of light output power on input power, directional characteristics and polarization characteristics were measured. Our method is very useful to realize electrically pumped edge emitting OSLs.