In this paper, a formal model for antivirus update agent system is presented based on mobile agent technology and predicate/transition Petri nets. The mobile agent system contains two mobile agents called DCA and UNA. It sends out agents to update antivirus on client computers in a network. Each agent takes on a specified responsibility. First, DCA roams through the network and check the last date of updating of antivirus on client computers. Then, by passing the list of unupdated client computers to UNA, next migration is started. The mobile agent system is modeled with logical agent mobility method (LAM) using Petri nets. Each agent is modeled with a predicate/transition Petri net. In this model, the antivirus updating system consists of a set of components to identify different locations and a set of connectors to specify the interactions among the components. Connectors and components are modeled with PrT Nets.