Extending the novel method of Eich et al. [J. Appl. Phys. 66, 3241 (1989)] to prepare highly stable second-order nonlinear optical (NLO) polymers via chemical cross-linking under electric field, we have obtained a new polymer exhibiting large and stable second-order optical nonlinearities after relaxation at 80°C. This was achieved by forming a network polymer from two NLO-active monomers, bifunctional N,N-(diglycidyl)-4-nitroaniline and trifunctional N-(2-aminophenyl)-4-nitroaniline. Here, every NLO moiety is connected to the network by a single covalent bond. After full cure under corona poling at 120°C, the sample exhibited at ambient conditions d33≅50 pm/V and d31≅16 pm/V at 1064 nm fundamental wavelength, as estimated from the Maker fringe data. Upon heating to 80°C, the nonlinearities decreased somewhat initially, but leveled off and remained stable at 80°C. This stable sample gave d33≅42 pm/V and d31≅14 pm/V at ambient conditions. Furthermore, the linear electro-optical coefficient of this sample, measured with a Mach-Zehnder interferometer, was r13≅6.5 pm/V at 530.9 nm. From this r13 coefficient one estimates d31≅11 pm/V at 1064 nm, in good agreement with the Maker fringe value.