The crosslinked polyurethane PU-VP was prepared by the reaction of poly[(phenyl-isocyanate)-co-formaldehyde] and hydroxy-functionalized hemicyanine-type NLO chromophore. For the poled and cured polymer theχ(2)value was 30.2 pm/V measured at a wavelength of 1.064 μm. Thermal stability studies performed on PU-PV sample by the second harmonic generation activity indicated a nearly no decay ofχ(2)value due to the lattice hardening of the polyurethane matrix with three bonding sites. The relaxation behaviors of aligned dipoles in PU-PV were also studied by varying poling and crosslinking conditions.