  We study dynamical effects of cosmics rays (CRs) on the thermal instability
in the linear regime. CRs and the thermal plasma are treated as two different
interacting fluids, in which CRs can diffuse along the magnetic field lines. We
show that growth rate of the magnetothermal condensation mode is reduced
because of the existence of CRs and this stabilizing effect depends on the
diffusion coefficient and the ratio of the CRs pressure to gas pressure. Thus,
a slower rate of structure formation via thermal instability is predicted when
CRs are considered.
