High nitrile terpolymers comprised of acrylonitrile (92 wt%), methyl acrylate (4 wt%), and hydroxylethyl methacrylate (4 wt%) (PAN) were synthesized and crosslinked using hexamethylene diisocyanate (HDI) terminated urethane prepolymer prepared from different molecular weights of poly(tetramethylene adipate) glycol (PTAd). The effects of urethane crosslinking and crosslinker length are studied. The degree of swell and soluble fraction of PAN increased dramatically and density decreased with crosslinker length. Wide angle X-ray diffraction showed an increase in the lateral repeat distance of hexagonal lattice as well as the crystalline size. The tan delta peak temperature of PAN decreased by crosslinking, however, the rubbery state extended with higher temperature, with the modulus (G(N)(o)) increasing as the crosslinker shortened. The undrawn crosslinked PAN showed ductile fracture with pronounced yielding and necking, whereas the undrawn linear PAN showed brittle fracture. The drawn linear PAN sample split into fibrils at the fracture tip, whereas the crosslinked PAN showed smooth fracture surfaces.