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.