The 3D virtual worlds (3DVWs) were widely used for language learning in the past two decades. Although numerous research studies acknowledged the affordance, a full-scale investigation that discussed how they can be applied to language learning with substantial evidence of effect sizes were not presented. Thus, this analysis aimed to synthesize the research findings particularly in the impact of 3DVWs on language learning by addressing two issues. The first one concerned the role of interaction in 3DVWs with theoretical bases, while the second issue concerned the effect sizes of moderating variables, based on experimental findings that were published between 2008 and 2019. Data with 50 variables generated from 13 primary studies were analyzed. Different patterns of interaction were identified based on three theories. The meta-analysis showed (1) significant overall linguistic gains (d = 0.832) and affective gains (d = 0.531), and (2) various effect sizes for moderator variables, such as types of learning activities. Based on these results, this paper used the notion of partnership to interpret the relation between 3DVWs and language learners.