An interpenetrating polymer networks （IPN） consisting of an epoxy-based polymer network and apolymethyl methacrylate network were synthesized and characterized. The IPN showed only one Tg, andhence a homogeneous-phase morphology was suggested. The second-order nonlinear optical coefficient (d33)of the IPN was measured to be 1.72×10-7 esu. The study of NLO temporal stability at room temperature andelevated temperature （100℃） indicated that the IPN exhibits a high stability in the dipole orientation due tothe permanent entanglements of two component networks in the IPN system. Long-term stability of secondharmonic coefficients was observed at room temperature for more than 1000 h.
