Different molecular weights of polyacrylonitrile (PAN) homopolymers were prepared based on the process of aqueous phase precipitation polymerization of acrylonitrile. Aqueous phase precipitation polymerization system includes water as aqueous media, acrylonitrile as monomer, ammonium persulfate as initiator, and drops of HCl as the activator of polymerization process. The polymerization processes were investigated at different monomer and initiator ratios, and different pH. The results showed that molecular weight of prepared polyacrylonitrile increased up to with increasing the monomer ratio at optimized pH 2.25, and the conversion percent was decreased. However, with increasing initiator ratios the opposite trend was occurred, i.e. the molecular weight decreased and conversion percent increased. The optimized conditions for getting suitable molecular weight at high conversion (84%) were at pH 2.25, 90 g/L of monomer concentration, and 0.67 g/L of APS initiator at time 6 hr. The prepared polymers were investigated using XRD, FTIR, H1NMR, TGA, and DSC. XRD results showed that crystallinity of the prepared polymers was decreased with increasing its molecular weight, as well as, the higher molecular weight of prepared PAN has more thermal stability than lower molecular weight.