This study pragmatically makes a descriptive analysis of metadiscourse in academic lectures from the perspective of the relevance-adaptation theory. Based on the relevance theory and the adaptation theory, the relevance-adaptation model is constructed to explore the occurrence, the pragmatic description and the role of metadiscourse in academic lectures. The data is collected from George Lakoff's 10 academic lectures on cognitive linguistics at Beijing University of Aeronautics and Astronautics in 2004 and some academic lectures audio-taped in classrooms. The results of the study show that 1) the occurrence of metadiscourse in academic lectures is pervasive; 2) the process of choosing appropriate metadiscourse is dynamic and the result of searching for relevance and making contextual adaptations – the process of searching for relevance of metadiscourse is measured in terms of three pairs of the distinctions, namely, contextual effect and processing effort, explicature and implicature, and conceptual meaning and procedural meaning, and contextual adaptations of metadiscourse are constrained by such factors as linguistic reality, psychological motivations and social conventions; 3) the role of metadiscourse in academic lectures is an active discourse constructor.