The first direct evidence for second-order nonlinearity in partially cross-linked polyacrylonitrile is reported. The cross-linking in gamma -ray-induced polymerized neat acrylonitrile is well supported by UV-vis spectroscopy, FTIR, and C-13 CP/MAS solid-state NMR. The second harmonic generation (SHG) powder test was employed to access the preliminary information about the occurrence of second-order nonlinear effect. The second harmonic generation is accomplished without the application of an external electric field, thus illustrating the importance of structural changes imparted by high-energy photon flux. Second-order nonlinearity in the present case is expected due to the small pi conjugation imparted by cross-linking through -C double bondN across polymeric chains coupled with the noncentrosymmetric molecular structure. Second harmonic intensity as high as 80% of that for powder urea has been observed.