  Based on an improved isospin- and momentum-dependent interaction, we have
studied the qualitative effect of isospin splitting of nucleon effective mass
on the specific shear viscosity of neutron-rich nuclear matter from a
relaxation time approach. It is seen that for $m_n^\star>m_p^\star$, the
relaxation time of neutrons is smaller and the neutron flux between flow layers
is weaker, leading to a smaller specific shear viscosity of neutron-rich matter
compared to the case for $m_n^\star<m_p^\star$. The effect is larger in nuclear
matter at higher densities, lower temperatures, and larger isospin asymmetries,
but it doesn't affect much the behavior of the specific shear viscosity near
nuclear liquid-gas phase transition.
