Using the spectra of the three M-components following a first return stroke recorded by a slitless spectrograph, the spectral features of the M-components are analyzed. Combining with plasma theories, the temperatures and the electron densities of the M-components in the channel core and outer corona sheath are calculated. The variations along the channel of these two parameters are studied, and compared with the corresponding return stroke. The results show that the spectra of the M-components are different from the spectrum of the return stroke. The optical radiation of the M-component is primarily from the spectral lines in infrared waveband. The temperature of the M-component in the channel core can reach 40000 K. The electron density of the M-component in the channel core is on the order of 10¹⁸ cm^–3. The temperature of the M-component in the external corona sheath is about 20000 K. The electron density of the M-component in the external corona sheath is on the order of 10¹⁷ cm^–3. The temperature of the M-component in the channel core decreases with height increasing, while that in the external corona sheath increases with channel height increasing. The electron density of the M-component in the channel core basically does not change with channel height. Whereas, the electron densities in the external corona sheath for two M-components with hard light at the upper end of the channel increase with channel height increasing, and the electron density for one M-component with weak light at upper end of the channel basically does not change with the channel height. By comparison, the temperature in the core channel and in the external corona sheath of the corresponding return stroke both increase with channel height. The electron density in the core channel and in the external corona sheath of the corresponding return stroke both basically remain constant along the channel.