Mobile agents are increasingly used in various Internet-based applications such as electronic commerce, network management, and information retrieval. If mobile agents can dynamically add, delete, and change their functionalities at run-time, they can sufficiently satisfy characteristics such as scalability, dynamicity, robustness, and performance, which are important in Internet-based applications. In this paper, we introduce Dynamic Composition of Functionality (DCF) based on code mobility, which enables mobile agents to dynamically compose their functionalities at run-time. In order to realize DCF based on code mobility. we propose necessary language constructs and implement a platform for scalable mobile agents, called DC-AOP. We also present a case study using DC-AOP to show the usefulness of our proposal.