This paper describes the development of a serious-game-based rehabilitation system aimed at encouraging stroke patients with upper limb motor disorders to practice physical exercises. The system contains a series of physically-based Virtual Reality (VR) games. A framework for physically-based VR rehabilitation systems is described which consists of a number of serious game designed to encourage patients' physical activity in highly motivating, physics-enriched virtual environments where various factors such as size and gravity can be scaled to adapt to individual patient's abilities and in-game performance. In addition, one goal of this study was also to determine whether physically-based VR games are an effective rehabilitation for training individuals who suffer from upper limb motor disorders as a result of a stroke. A pilot study with 8 participants who have a first hemispheric stroke shows improvements on impairment measurement and functional measurement shortly after completion of the intervention and 6 weeks after the intervention. Despite several limitations in this study the results support the effectiveness of physically-based VR games in the treatment of participants with hemiplegia and further development of the serious games.