Knowledge sharing in computer supported collaborative learning (CSCL) requires intensive social interactions among participants, typically in the form of annotations. An annotation refers to an explicit expression of knowledge that is attached to a document to reveal the conceptual meanings of an annotator's implicit thoughts. In this research, we develop a Semantic Web and Web services-supported multimedia tool to facilitate collaborative annotation sharing in the context of CSCL. Our experimental results demonstrate that our tool can facilitate knowledge sharing and improve participating students' reading comprehension: it helps participants to raise good questions and provide proper answers through the practice of reading, commenting, reviewing, and discussion. (Contains 1 table and 8 figures.)