  Inspired by the recent observation of the $D^0-\bar D^0$ mixing, we explore
the effects of colored scalars on the $\Delta C=2$ process in diquark models.
As an illustration, we investigate the diquarks with the quantum numbers of (6,
1, 1/3) and (6, 1, 4/3) under $SU(3)_C\times SU(2)_L\times U(1)_Y$ gauge
symmetries, which contribute to the process at one-loop and tree levels,
respectively. We show that $\Delta m_D$ gives the strongest constraint on the
free parameters. In addition, we find that the small couplings can be naturally
interpreted by the suppressed flavor mixings if the diquark of (6, 1, 4/3) only
couples to the third generation.
