For enhancing the compatibility and/or the interpenetration of the simultaneous interpenetrating networks (SINs) composed of polyurethane (PU) formed with uretonimine modified 4,4'-diphenylmethane diisocyanate and vinyl ester resin (VER), a series of graft VERs consisting of different lengths of side chains were synthesized and characterized. It was found that there exists some limited short-range order due to the strong hydrogen bonding in the graft VER network composed of butanol side chains (BO-g-VER). The graft VER network composed of poly(oxypropylene) (PPO) side chains (M-n: 200, 200-g-VER) showed compatible system, while the VER network consisting of longer PPO grafts (M-n: 390, 390-g-VER) exhibited microphase separated morphology. Based upon the DSC and FTIR measurements as well as the SEM and TEM observation, the lengths of side chains existing in graft VER network have great effect on the morphologies of PU/graft VER SINs. For PU/BO-g-VER SINs, there has been some interpenetration between the two networks because of the miscibility between the BO-g-VER network and the hard segments existing in the PU network. For PU/200-g-VER SINs, the good compatibility and/or the interpenetration between the two phases was observed, since the long-range ordered structure of hard segments in PU phase was greatly suppressed, resulting from the excellent miscibility between the urethane groups as well as the PPO side chains existing in the 200-g-VER network and those in the PU network, respectively. Thus, the strong reinforcement effect of these two graft networks on the PU network and the excellent mechanical properties of the SIN systems were observed. However, the PU/390-g-VER SINs showed the complicated morphologies because of existing microphase-separated morphology of 390-g-VER network in itself. In this case, the enhancement effect of such a graft VER network on the PU network is limited.