Research on organizational improvisation has gained much momentum in information systems development literature. Although recent studies have started to explore how to conduct improvisation effectively and generated rich findings on the antecedents, the process of effective improvisation remains unknown and the lack of knowledge in this area may account for the difficulties faced by many IS practitioners in engaging in effective improvisation. Based on a case study on Tencent’s software product development, we derive a four-phase process model of effective improvisation, which is a continuous iteration between improvisational search and build. The former consists of simultaneous grounded observation and situated reflection. The latter consists of simultaneous grounded design and situated execution. The findings contribute to both the IS and improvisation literature, and transform into practical implications for IS managers, who are increasing relying on real-time planning and spontaneous execution.