Poly (urethane-epoxy)/allyl novolac resin simultaneous interpenetrating networks (SINs) were prepared by crosslinking urethane-epoxy adduct with diamino diphenylmethane (DDM) and allyl novolac resin simultaneously. An urethane-epoxy adduct was synthesized by terminating the urethane prepolymer composed of 4,4’-diphenyl methane diisocyanate (MDI) and poly (tetramethylene oxide) (PTMO), whose molecular weight range falls in 600-700 (UT1), 900-1050 (UT2) and 1900-2100 (UT3) with glycidol (GL). The allyl novolac resin was derived from novolac type phenolic resin by substituting the hydroxy group on the aromatic ring with the allyl group. The thermal, mechanical and dynamical characteristics of these SINs were measured by differential scanning calorimetry (DSC), Instron tester and dynamical mechanical analysis (DMA), and correlated well enough with morphological observations from transmission electron microscopy (TEM). All these SINs exhibit a two-phase structure and a reasonable explanation is provided for the correspondence of the phase mixing and the specific mechanical properties.