  We adopt the chiral perturbation theory to calculate the
$\Sigma_{c}^{(*)}\bar{D}^{(*)}$ interaction to the next-to-leading order (NLO)
and include the couple-channel effect in the loop diagrams. We reproduce the
three $P_c$ states in the molecular picture after including the
$\Lambda_{c}\bar{D}^{(*)}$ intermediate states. We also discuss some novel
observations arising from the loop diagrams.
