By introducing an additional loss parameter for the description of SCC and developing a model of modified multimode rate equations, the mode selection mechanism and characteristics of SCC semiconductor lasers have been investigated theoretically. It is found that an antireflection coating on the end facet of a laser diode is necessary for high-quality single longitudinal mode operation. Based on the theoretical results, a new type of miniature SCC laser has heen constructed and demonstrated. Single mode outputs with 35~40 dB side mode suppression ratio have been obteined under CW and 150 MHz modulation conditiona.