Examining the learner agency expressed by two learners of Chinese as a second language (CSL), the current two-case study was conducted from the perspective of language socialization. Qualitative data have been collected through classroom observations and semi-structured interviews. Data analysis reveals that the two cases in the current study exhibited eight forms of learner agency in their CSL learning, including self-determination, self-motivation, self-regulation, participation, cooperation, negotiation, hesitancy and resistance. In addition, learner agency observed in the two cases is constantly mediated by multiple sociocultural factors in specific contexts of CSL socialization. The results of the current study imply the socioculturally mediated essence of learner agency and suggest that learner agency develops through a temporal course and is constrained by social structures.