A multi-band antenna for a portable communication device is disclosed, in which the communication device includes a first housing, a second housing and a substrate. The multi-band antenna includes a feeding portion, a system ground plane, a metal ring, a resonant cavity, a first and a second radiating portion. The system ground plane is disposed on the substrate. The metal ring is connected to the first housing, and forms a space with the first housing to accommodate the substrate, in which the metal ring is electrically coupled to the system ground plane through a plurality of ground ends. The resonant cavity is formed between the system ground plane and the metal ring to generate a first resonant mode. The first and the second radiating portion are disposed on the second housing, for generating a second and a third resonant mode, respectively.