In recent years, metaphor has become a focus of interest in cognitive linguistics. The researchers hold that metaphor is a fundamental way of human cognition rather than just a rhetorical device in traditional view. In this paper, three traditional theories of metaphor (the substitution theory, the comparison theory and the interaction theory) are briefly reviewed and discussed. Totally different from them, cognitive linguistics holds a brand-new philosophical ground-experientialism and maintains that metaphor is pervasive in thought and action as well as in language.In order to clearly locate the metaphorical concept in the panorama of human cognition, the theoretical framework of cognitive semantics is briefly discussed. The mechanism of conceptual metaphor consisting of matching and similarity, and four features of metaphor are examined.As a case study, this paper intends to put forward a kind of method in lexical comparison and lays emphasis on applying LakofTs classification of metaphors to the analysis of the time concept underlying Chinese and English lexicons. Lakoff s classification, including orientational, ontological and structural metaphors, is introduced and ways of applying his classification to this study are discussed.The analysis shows that Chinese and English lexicons are really a wealth of data under the cover of which metaphorical concept of time is pervasive and can be traced back to a limited set of clusters. Those main conceptual metaphors concerning time are classified and explained with samples from English and Chinese lexicons and the differences in understanding them are disclosed through the analysis of those relevant lexemes. Finally, the causes of the differences are explored and the implication of this kind of lexical comparison on the level of cognition is discussed.