Different nanostructure shapes of polyaniline (PAni) particles such as rod and spherical shapes were prepared using gamma radiation. Gamma radiation is advantageous over other conventional methods because it provides a pure nanoparticle without by-products or any additives. The four solutions of aniline monomer Ani were dissolved in aprotic polar solvent N,N-dimethyl formamide (DMF) in different pH values and exposed to gamma radiation at a dosage of 50 kGy. The pH effect in terms of size and structure of PAni was investigated with four values 1, 5, 8 and 13 adjusted by adding HCl. A common polymerization mechanism induced by radiation is proposed as a radical cation polymerization, which is an environment friendly and cost effective mechanism without any side products or contaminants. The size and size distribution of PAni NPs suspension in DMF solvent were investigated by TEM and DLS. The results clearly showed that the ionizing radiation is an effective tool to produce a shape-controllable size. The changes of the spectrum with the pH of the polymerization solution obtained from UV-visible spectroscopy revealed that the smallest particles of PAni NPs can easily be obtained at low pH. The PAni particles are obtained with two morphologies (such as rod and spherical shapes).