  We investigate viscous effects on the dynamical evolution of QCD matter
during the first-order phase transition, which may happen in heavy-ion
collisions. We first obtain the first-order phase transition line in the QCD
phase diagram under the Gibbs condition by using the MIT bag model and the
hadron resonance gas model for the equation of state of partons and hadrons.
The viscous pressure, which corresponds to the friction in the energy balance,
is then derived from the energy and net baryon number conservation during the
phase transition. We find that the viscous pressure relates to the
thermodynamic change of the two-phase state and thus affects the timescale of
the phase transition. Numerical results are presented for demonstrations.
