  Velocity fields of molecular clouds (MCs) can provide crucial information on
the merger and split between clouds, as well as their internal kinematics and
maintenance, energy injection and redistribution, even star formation within
clouds. Using the CO spectral lines data from the Milky Way Imaging Scroll
Painting (MWISP) survey, we measure the relative velocities along the line of
sight ($\Delta$V$_{\rm LOS}$) between $^{13}$CO structures within $^{12}$CO
MCs. Emphasizing MCs with double and triple $^{13}$CO structures, we find that
approximately 70$\%$ of $\Delta$V$_{\rm LOS}$ values are less than $\sim$ 1 km
s$^{-1}$, and roughly 10$\%$ of values exceed 2 km s$^{-1}$, with a maximum of
$\sim$ 5 km s$^{-1}$. Additionally, we compare $\Delta$V$_{\rm LOS}$ with the
internal velocity dispersion of $^{13}$CO structures ($\sigma_{\rm
^{13}CO,in}$) and find that about 40$\%$ of samples in either double or triple
regime display distinct velocity discontinuities, i.e. the relative velocities
between $^{13}$CO structures are larger than the internal linewidths of
$^{13}$CO structures. Among these 40$\%$ samples in the triple regime, 33$\%$
exhibit signatures of combinations through the two-body motion, whereas the
remaining 7$\%$ show features of configurations through the multiple-body
motion. The $\Delta$V$_{\rm LOS}$ distributions for MCs with double and triple
$^{13}$CO structures are similar, as well as their $\Delta$V$_{\rm
LOS}$/$\sigma_{\rm ^{13}CO,in}$ distributions. This suggests that relative
motions of $^{13}$CO structures within MCs are random and independent of cloud
complexities and scales.
