The present article examines peculiarities of realization and functioning of conceptual metaphor in scientific and popular science discourse based on the material of German. A case study of metaphor KÖRPER IST MECHANISMUS (“Body is a mechanism”) dwells on establishing of a correlation between function of conceptual metaphor and the discourse it is used in. The relevance of the research is based on the interest in the study of the connection between thinking and language und the role of the metaphor in the process of categorization, conceptualization and structuring of new knowledge. Metaphor enables to explain one fact through another by the means of analogy or similarity and acts as an instrument to perception of complex abstract phenomena and processes. Novelty of the approach results in comparative character of analysis of functions that are peculiar to metaphor in stringent scientific and in derivate popular science discourse. In the course of the study, it has been discovered that metaphors in scientific texts serve the purpose of creating new concrete meaning by the means of understanding one conceptual field through the other, while metaphors in popular science texts are used to explain complex propositions by the means of already known to recipient. Conceptual metaphor is thus an instrument of nomination and serve the heuristic function in scientific discourse. On the other hand, it possesses of more expressivity and fulfills the didactic function in popular science discourse. The obtained results make contribution to the theoretical cognitive linguistics and can be used to study metaphor in other types of discourse. The research perspectives include contrastive analysis of functions of conceptual metaphor in German and other European languages.