Second-order nonlinear optical copolymers containing p-hydroxyphenyl maleimide or 4-cinnamoyloxy-phenylmaleimide as a comonomer were newly synthesized. For improving the temporal stability, we adopted the crosslink reaction thermally and photochemically. Second-order nonlinear optical properties of the poled/cured films were investigated in terms of the second harmonic generation (SHG) and linear electro-optic (EO) coefficient measurement. SHG coefficient, d(33) was found to be 14 pm/V at the fundamental wavelength of 1064 nm and EO coefficient, r(33) was found to be 25-30 pm/V at 633 nm wavelength in two different crosslinked structures. To prevent the orientational relaxation of poled polymer, thermal crosslink reaction was induced using diisocyanate as a crosslinker between the side chains themselves. Another photo-crosslink was induced exposing the film at 257 nm wavelength during poling. Temporal stabilities of second-order NLO coefficients of crosslinked polymer systems were proved much better than that of guest-host system or other side chain polymers.