  The bubble wall velocity in the first order phase transition plays an
important role in determining both the amplitude and the pivot frequency of
stochastic gravitational wave background. In the framework of the minimal
left-right symmetric model, we study the wall velocity when the first order
phase transition can occur. The wall velocity can be determined by matching the
distribution functions in the free particle approximation and the local thermal
equilibrium approximation. It is found that the wall velocity can be determined
in the range $ 0.2 < v_w < 0.5 $ for the parameter space with the first order
phase transition. It is also found that for the case when the wall velocity is
close to the speed of sound, the peak amplitude of gravitational wave spectrum
can be larger than that in the runaway case. Moreover, It is also found that
there exists an approximate power law between the wall velocity and pressure
difference between broken and symmetry phases, and the power index is equal to
0.41 or so.
