  We investigate the probability distribution of the effective inspiral spin,
the mass ratio, and the chirp mass of primordial black hole (PBH) binaries,
incorporating the effect of the critical phenomena of gravitational collapse.
As a leading order estimation, each binary is assumed to be formed from two
PBHs that are randomly chosen according to the probability distribution of
single PBHs. We find that, although the critical phenomena can lead to large
spins on the low-mass tail, the effective inspiral spin of the binary is
statistically very small,
$\sqrt{\langle\chi_{\mathrm{eff}}^2\rangle}=8.41\times10^{-4}$. We also see
that there is almost no anti-correlation between the effective inspiral spin
and the mass ratio, which can be inferred from observations.
