Stimulated emission was studied in dendrimer-doped polymer waveguides. A supernarrowing laser spectrum was observed without using any resonator mirrors, due to the moderate scattering caused by densely doped dendrimer aggregates in a polymer waveguide. At the optimum operation level, the spectral linewidth was found to be 0.55 nm, which was limited by the resolution of the spectrograph used. Weak scattering from the aggregated dendrimers in the host polymer matrix with gain was responsible for the photon localization. However, in contrast to conventional random lasers with strong scattering, weak scattering enables the long amplification path through the waveguide producing a directional beam and a supernarrowing spectrum. The dependence of supernarrowing lasing behavior on the density of the scattering moiety was studied by changing the dendrimer doping ratios in the polymer matrix,