Second-order nonlinear optical copolymer containing p-hydroxyphenyl maleimide as a comonomer was newly synthesized. Second-order nonlinear optical properties of the poled films were investigated in terms of the second harmonic generation (SHG) and linear electrooptic (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. To prevent the orientational relaxation of poled polymer, the thermal crosslink reaction was induced using diisocyanate as a crosslinker between the side chains themselves. 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.