Radiation polymerization of acrylonitrile in a viscous system with Styrene was performed at ambient temperature by using gamma-rays. It is found that the overall rate of polymerization was accelerated after critical conversion due to the gel effect. As the molar fraction of styrene in monomer feed (f(St)) is increased, both the total polymer conversion and molar fraction of acrylonitrile in the copolymer feed (F-AN) were decreased. The monomer reactivity ratios for acrylonitrile and styerne were determined to be r(1) (AN) = 0.25 and r(2) (St) = 2.0, respectively. The copolymers obtained were characterized by Fourier transformed infrared spectra (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), H-1-NMR, and pyrolysis mass spectrometry (PMS). It was found that the slight addition of styrene to acrylonitrile strongly changes crystallinity, morphology, and thermal decomposition of the resulting polymer. H-1-NMR measurement of AN/St copolymer showed the appearance of aromatic proton signals and shifted the resonance of the methylene proton to lower chemical shifts. The mass spectra of AN/St copolymers showed fragments of pyrolysates corresponding to oligonitriles with styrene end groups.