Feasibility of repeaterless transmission systems over non-dispersion-shifted fiber (NDSF) using directly modulated DFB lasers and dispersion compensating fiber (DCF) was examined. The transmission characteristics were investigated by simulations, considering the effects of the laser chirping, fiber chromatic dispersion, and the self-phase-modulation (SPM) occurring in both NDSF and DCF. As a result, it was revealed that the post-compensation scheme is the most suitable and that the SPM in DCF dominantly deteriorates the transmission characteristics in this scheme. Following the predictions by the simulations, 2.5 Gbit/s repeaterless transmission over 306-km pure-silica-core fiber has been demonstrated with no power penalty, using a directly modulated DFB laser with a bulk active layer and dispersion compensating fibers.