Spontaneous emission factor β is an important parameter to characterize semiconductor optic-electronic devices. In characteristic analysis of the superluminescent diodes, especially, in calculation of optical intensity using the rate equations, the value of the factor β estimated in the laser diodes is commonly applied since the superluminescent and laser diodes are similar in the structures. However the two types of devices are quite different, the extended usage of the factor may lead to confusion in physical concepts and make great error. In this paper, the spontaneous emission factor β for the superluminescent diodes is discussed, and a new concept on average value of the laser β is proposed. From estimation and comparison, the factors β of gain-guide and index-guide structures are obtained.