Abstract According to the structures traced by ¹³CO spectral lines within ¹²CO molecular clouds (MCs), we investigate the contributions of their internal gas motions and relative motions to the total velocity dispersions of ¹²CO MCs. Our samples of 2851 ¹²CO MCs harbor a total of 9556 individual ¹³CO structures, among which 1848 MCs (∼65%) have one individual ¹³CO structure and the other 1003 MCs (∼35%) have multiple ¹³CO structures. We find that the contribution of the relative motion between ¹³CO structures ( σ 13 CO , re ) is larger than that from their internal gas motion ( σ 13 CO , in ) in ∼62% of 1003 MCs in the “Multiple” regime. In addition, we find that σ 13 CO , re tends to increase with the total velocity dispersion ( σ 12 CO , tot ) in our samples, especially for MCs with multiple ¹³CO structures. This result provides a manifestation of macroturbulence within MCs, which gradually becomes the dominant way of storing kinetic energy along with the development of MC scales.