Advances in wireless network technology and the increasing use of number of handheld mobile devices make many services and applications necessary to be offered to mobile users. An event notification service (ENS) system is an extremely useful communication service for those applications. Therefore, it becomes necessary to extend ENS system to a mobile environment. A distributed ENS system is an asynchronous system consisting of several subsystems, namely, producer clients, consumer clients and servers without common memory and global clock. The subsystems communicate each other by sending messages with unpredictable transmission delays causing unordered propagation of messages. In this paper we describe a model of ENS system for mobile environment. Then we present a causal ordering algorithm of ENS systems for mobile environment.