  According to the structures traced by $^{13}$CO spectral lines within the
$^{12}$CO molecular clouds (MCs), we investigate the contributions of their
internal gas motions and relative motions to the total velocity dispersions of
$^{12}$CO MCs. Our samples of 2851 $^{12}$CO MCs harbor a total of 9556
individual $^{13}$CO structures, among which 1848 MCs ($\sim$ 65$\%$) have one
individual $^{13}$CO structure and the other 1003 MCs ($\sim$ 35$\%$) have
multiple $^{13}$CO structures. We find that the contribution of the relative
motion between $^{13}$CO structures ($\sigma_{\rm ^{13}CO, re}$) is larger than
that from their internal gas motion ($\sigma_{\rm ^{13}CO, in}$) in $\sim$
62$\%$ of 1003 MCs in the `multiple' regime. In addition, we find the
$\sigma_{\rm ^{13}CO, re}$ tends to increase with the total velocity
dispersion($\sigma_{\rm ^{12}CO, tot}$) in our samples, especially for the MCs
having multiple $^{13}$CO structures. This result provides a manifestation of
the macro-turbulent within MCs, which gradually becomes the dominant way to
store the kinetic energy along with the development of MC scales.
