Mechanism and condition of single longitudinal mode operation of the semiconductor lasers with external dispersive cavity have been investigated theoretically and experimentally. Using multimode laser diodes with unooating facet as well as coating facet, stable single longitudinal mode is obtained under room temperature CW opera- tion. Its linewidth is less than 10-3 A(50 MHz) and the tunable range is 200 A. Single mode output power is more than the total power output of multimode with our external cavity up to 3 mW.