Agents are becoming one of the most important topics in distributed and autonomous decentralized systems (ADS), and there are increasing attempts to use agent technologies to develop software systems in electronic commerce. Such systems are complex and there is a pressing need for system modeling techniques to support reliable, maintainable and extensible design. G-Nets are a type of Petri net defined to support modeling of a system as a set of independent and loosely-coupled modules. In this paper, we first introduce an extension of G-Net, agent-based G-Net, as a generic model for agent design. Then new communication mechanisms are introduced to support asynchronous message passing among agents. To illustrate our formal modeling technique is effective for agent modeling in electronic commerce, a price-negotiation protocol example between buyers and sellers is provided. Finally, by analyzing an ordinary Petri net reduced from our agent-based G-Net models, we conclude that our agent-based G-Net models are L3-live, concurrent and effective for agent communications.