The electrocatalytic oxidation of ascorbic acid(AA) in H2SO4 solution at polyaniline(PAN) modified electrode was studied by cyclic voltammetry(CV). The electrocatalytic behavior was affected by the PAN film thickness(D), the pH values of solution, the AA concentration and the metalic cation. The results show that the peak current has od linear relationship with the AA concentration at pH=4.5, D=0.1μm in 1o-2 ~10-8mol?L-1 AA. The electrocatalytic activity was strengthened by Cu2+ and Fe2+ ions. The CV behavior was also investigated under the magnetic fields up to 0.7T(1T=104 Gauss). Plateaus of limiting currents were observed when the magnetic flux was directed in parallel to the surface of electrode. The phenomena were explained by the enhancement of mass transfer by magnetohydrodynamic (MHD) convection. The magnetic field effects was found to be pronounced at the slow potential scan rate of 10mv?s-1. But when the magnetic flux directed in perpendicular to the surface of electrode, the effect was not observed.