The growth process of polyaniline (PANI) nanofibers during the electrochemical polymerization was investigated in detail. The nano-fibrillar morphology appears to be intrinsic to PANI, and the unique character is attributable to a combined effect of electrophilic substitution reaction mainly taking place at the para-position of aniline or its oligomers and aniline oligomers with one-dimensional (1D) structure. Interestingly, the PANI film formed on the electrode exhibits a lamellar structure with compact two-dimensional (2D), micro-granular, nanorod-shaped, and nano-fibrillar PANI layers from bottom to top. In addition, the possible formation mechanism of the lamellar structure of PANI film is discussed.