Five reaction channels of the title reaction have been studied with the second-order Moller-Plesset perturbation theory by use of a split valence plus polarization 6-31G** basis set. The calculated results show that among the five channels the title reaction may favor the aza-Wittig and the addition routes. The aza-Wittig reaction H3PNH + H2CO --> H3PO + H2CNH is shown as a two-step reaction via the first transition state, a four-membered ring intermediate, and then the second transition state. The first and the second forward barriers are 12.2 and 18.0 kcal/mol, respectively. The addition reaction H3PNH + H2CO --> PH2NHCH2OH possesses only one transition state with a barrier of 13.3 kcal/mol.