By introducing terms describing the random frequency modulation into optical Bloch equation

we have discussed spectral diffusion in such systems after a narrow band excitation. Especially

1) if ω jumps to any other ω′ in the ensemble with equal probability

the time dependent lineshape would be a superposition of an exponentially decayed localized line and an exponentially grown background; 2) if the probability that ω jumps to ω′ depends on |ω′-ω|

the delocalized peak would be broadened with time. We have also discussed spectral diffusion in the system with frequency modulated by numbers of independent random telegraph processes as an example.