The observed N-regioselective glycosidation of 2-O-substituted 5-fluorouracil (5-FU) via the phase-transfer-catalysis(FTC) method was investigated computationally. The Gibbs free energy reaction barrier of the N-reaction between the 5-FU anion and 1-bromo-1-deoxy-2,3,4,6-tetra-O-acetyl-alpha-n-glucopyranose was computed at the MP2/6-311++G(2d,p)//B3LYP/6-31+G* level. The calculated transition states were, in general, quite "loose", with the ambident reaction sites at the N3- or 04-positions on 5-FU located approximately 2.0 angstrom from the anomeric carbon. With the S(N)2 mechanism, the formation of, beta-glycosides was explained by the characteristics of transition states, and the N-regioselectivity was explained by three considerations: (1) the conformations of initial complexes and the structural requirement of the reactions; (2) the formation of an ionic pair between nBu(4)N(+) and 2-O-substituted 5-FU anions; and (3) the thermodynamic conversion of O-glycosides to N-glycosides. The reactions between the oxocarbenium ion and the 2-O-substituted 5-FU anions (the fast step of S(N)1 mechanism) were also examined at the same level of theory. Because there were no "promoters" to extract Br in the PTC method, the S(N)1 mechanism might have an unfavorably high barrier to produce oxocarbenium ion. However, both the formation of beta-glycosides and the experimentally observed N-regioselectivity could also be explained by the S(N)1 mechanism: The former was explained by the neighboring group participation, and the latter was explained by the formation of ionic pairs between nBu(4)N(+) and 2-O-substituted 5-FU anions. The formation of ionic pairs possibly changed the diffusion-controlled mechanism into an activation-controlled mechanism Two factors were demonstrated by Marcus theory to play an important role for the experimentally observed N-resioselectivity in the PTC method: (1) the thermodynamic stability of N-products over O-products; (2) the formation of ionic pair between nBu(4)N(+) and 2-O-substituted 5-FU anions.