A self-crosslinkable second-order nonlinear optical copolymer with thermal stability at 100 degrees C was prepared by the copolymerization of methacrylic nonlinear optical monomer with glycidyl methacrylate. The nonlinear optical property of this copolymer was compared with that of well-known side-chain polymer P2ANS. Electro-optic coefficient and second harmonic generation measurements showed a r(33) value of 57 pm/V at 632.8 nm and a d(33) value of 45.5 pm/V at 1064 nm. Moreover, self-crosslinking during poling was observed between the epoxy groups in the copolymer without adding any crosslinking agent and thus the copolymer showed an excellent thermal stability of aligned chromophores at 100 degrees C. It is thought that the crosslinking was performed between the epoxy groups due to the catalytic basicity of tertiary amine groups in the NLO monomer.