Using the rate equation, several optical methods of characterizing the threshold of a laser diode (LD) were analyzed. The thresholds determined by all methods are consistent if the spontaneous-emission-coefficient (β) is small. If β>0.05, the thresholds obtained by different methods are different. Especially, for micro-nano LDs with a large β, these methods are starting to become inaccurate. However, the d2S/dI2-I is a relatively accurate method to characterize threshold of these LDs compared to other methods. The effects of the spontaneous-emission-lifetime (τsp) and the photon- lifetime (τp) on thresholds were analyzed. The exact functions between the threshold and the β, τsp and τp were obtained.