  We study the effects of velocity dispersion on the formation of primordial
black holes~(PBHs) in a matter-dominated era. The velocity dispersion is
generated through the nonlinear growth of perturbations and has the potential
to impede the gravitational collapse and thereby the formation of PBHs. To make
discussions clear, we consider two distinct length scales. The larger one is
where gravitational collapse occurs which could lead to PBH formation, and the
smaller one is where the velocity dispersion develops due to nonlinear
interactions. We estimate the effect of the velocity dispersion on the PBH
formation by comparing the free-fall timescale and the timescale for a particle
to cross the collapsing region. As a demonstration, we consider a log-normal
power spectrum for the initial density perturbation with the peak value
$\sigma_0^2$ at a scale that corresponds to the larger scale. We find that the
threshold value of the density perturbation $\tilde \delta_{\rm th}$ at the
horizon entry for the PBH formation scales as $\tilde \delta_{\rm th}\propto
\sigma_0^{2/5}$ for $\sigma_0\ll1$.
