After endovascular coil embolization of cerebral aneurysms, coil compaction and late aneurysm recanalization have been ever observed. The HydroCoil Embolic System (HES) was developed to improve the packing efficacy of endovascular treatment of cerebral aneurysms. In this study, we evaluate the packing efficacy of HES using a silicone model of ruptured cerebral aneurysm. This silicone model was connected to a pulsatile flow pump and embolized with the initial framing coils followed by hydro coils (n = 3) or bare platinum coils (n = 3). The coils used in the two groups were identical to each other in size and length. In the hydro coil group, continuous outflow from ruptured aneurysm ceased in two out of three cases. On the other hand, in the bare platinum coil group, outflow from the ruptured point slightly decreased but did not stop in all cases. The hydro coil could result in a higher initial occlusion rate of silicone model. In addition, expanded hydrogel possibly sealed the ruptured point directly. Changes in the size of aneurysms were not detected, from which the risk of over-expansion seemed extremely low. The hydro Coil is a safe and feasible device for improving the packing efficacy in endovascular coil embolization.