We present the formation of nano-structures especially nano-rod and nano-particles of polyaniline (PANI) using Langmuir-Blodgett (LB) technique of molecular deposition. Decrease in the pH of the sub-phase of LB trough increases level of doping (protonation) of polyaniline emeraldine base to emeraldine salt and causes doping induced nano-structure formation. High resolution transmission electron microscopy reveals that anisotropic rod like nano-structures of PANI of diameter less than 50 nm are formed at pH 1 while pH 3 induces the formation of dispersed nano-particle of diameter 15 nm. On the contrary, connected lump (aggregation) type nano-structures of PANI are formed at a higher value of pH 6. © 2008 Elsevier B.V. All rights reserved.