This research-in-progress paper reports on the development of an instrument for measuring flow experience of computer game players. Flow theory (Csikszentmihalyi, 1993) has been widely adopted in various research fields such as information systems (IS), human-computer action (HCI), and computer games. We argue that flow experience in computer game play leads to enjoyment and therefore can be measured as a dimension of enjoyment in addition to emotion. Development process of the instrument is discussed in this paper.