Abstract: Capability is an important abstract concept to specify and design agent, and to investigate agent’s computing. In the paper here, a capability theory of agent’s computing is put forward based on the internal perspective in support of the research on the theory of agent computing. The concept of plan tree is presented to describe how agent organizes its plans effectively to achieve its tasks in a non\|deterministic and dynamic multi\|agent system. Based on the logic framework of multi\|agent computing, the formal semantics of the capability is defined and some important logical properties are obtained.