This study provides a conceptual framework for the informal learning process in an online community. We conducted in-depth interviews with 11 community members and analyzed the data based on grounded theory. The study results are as follows: First, the causal conditions are ‘high intellectual growth desire, competent group members, interesting academic field, recommendation from trusted members,’ and the contextual condition is ‘lack of learning opportunities.’ Second, the core phenomenon is ‘intellectual play and reflection by observation.’ Third, the intervening conditions are a ‘humane atmosphere, unconscious sense of responsibility,’ and the strategies for action and interaction are ‘sustainability and transfer of learning, questions that provoke joint participation, fair regulation of speaking rights.’ Fourth, we analyzed the outcomes as ‘holistic growth experience, synchronization of passion, serendipitous insight, and abundant mindset.’ Based on the selective coding, the informal learning process consists of three stages: ‘enthusiasm and respect atmosphere’ for the early stage, ‘play and reflection’ for the mid stage, and ‘growth experiences and abundant mindset’ for the late stage. The core category is integrated as ‘limitless learning by intellectual play’. We discuss practical suggestions for informal learning in the online community.