Mie suppose that nonthermal spectra of radiation in gamma-ray bursts (GRBs)
are formed by optically thick spherical plasma shells that expand with rel
ativistic velocities. The temperature of the shell decreases as power law.
We discuss results of calculations which show that the radiation spectrum o
f such shell is similar to the observed spectra of GRBs. This model allows
to define some characteristics of emitting regions.