Superplastic behavior of the Zr(65)Al(10)Ni(10)Cu(15) glass metallic alloy produced by powder-metallurgy method was examined in the supercooled liquid region. Stress-strain and stress-strain rate relationships showed that Newtonian viscous flow governed the plastic flow until strain hardening took place. The large strain hardening was proved to a result of occurrence of crystallization during deformation.