The UPnP technology is widely employed to enable networked devices interconnected in home computing environments. The UPnP eventing protocol on top of HTTP over TCP performs in a unicast manner, however, suffering from scalability problem and performance degradation against highly frequent event notifications. This paper devises an efficient event multicast mechanism to mitigate above circumstances. The proposed mechanism can group service subscribers of the same access interest and synchronously inform all subscribers in a group of event notifications through a specific administratively scoped multicast delivery. Its design is compliable with GENA specification and so is able to coexist with the original UPnP eventing method. The experimental prototype shows that this mechanism is lightweight and does not complicate the UPnP system design.