Cyber Composer is a novel and interactive cyber instrument that enables both musicians and music laypersons to dynamically control the tonality and the melody of the music that they generate/compose through hand motion and gestures. Cyber Composer generates music according to hand motions and gestures of the users in the absence of real musical instruments. Music theories are embedded in the design so that melody flow and musical expressions like the pitch, rhythm and volume of the melody can be controlled and generated in real-time by wearing a pair of motion-sensing gloves. Also central to the design is the mapping of the hand motions and gestures to musical expressions that is intuitive and requires minimal training. Cyber Composer is expected to find applications in the fields of performance, composing, entertainment, education as well as psychotherapy.