Polyaniline (PANI) film formed under the effects of the magnetic field was characterized. In situ conductivity measurements with four microprobe electrodes were performed in acid aqueous solution. The electroactivity and potential dependent conductivity of the polymer film are much improved by use of a magnetic field. The results show that the upper value of conductivity for the PANI (B-p = 0.7 T) film prepared in the presence of the magnetic field parallel to the electrode surface is increased by a factor of three times that of a PANI film formed in the absence of the magnetic field, and the film has some anisotropic properties with a conductivity increase of 30% in the direction perpendicular to the magnetic flux compared with that in the parallel direction. The potential of the maximum conductivity is shifted negatively by 50 mV. The X-ray diffraction pattern of the polymer shows that the increase in crystallinity and structural order result from the magnetic alignment. It is suggested that the resulting polymer film formed under the magnetic field has a more ordered structure and better stability.