The beta-OH group is more active than gamma-OH in many processes, such as the catalysis of the active site in enzymes and the N-->O rearrangement of N-phosphoryl amino acids in negative-ion electrospray ionization mass spectrometry. The pentacoordinate phosphoric intermediates are proposed and the rearrangement mechanisms are studied by ab initio and density functional calculations. The formation of the intermediates comprises a concerted route and two different stepwise pathways. The concerted route is more favored. The cleavage of the P-N bond is the rate-determining step and is favored by the proton transfer. For the rearrangement of dimethoxyl phosphoryl serine anion and homoserine anion, the activation energies are 91.2 and 156.9 kJ/mol at the B3LYP/6-31+G(d,p) level, respectively. Therefore, the beta-OH group is predicted to be more active than gamma-OH and the result is in good agreement with the experiments.