Le is a functional morpheme in Mandarin, which can appear in two places, immediately after the verb (verb-le) or in a sentence-final position (sentence-le). Traditionally, verb-le is often referred to as a perfective aspect marker denoting completion, while sentence-le is generally considered as a sentence final particle which signals a change-of-state meaning. Based on Smith’s aspect theory, which calls the grammatical aspect the viewpoint aspect and the lexical aspect the situation aspect, this paper argues that both les are perfective aspect markers derived from one super-le. Besides, it also compares le with guo, which is another post-verbal perfective aspect marker whose aspectual status has been well-established in the literature, and points out that guo differs from verb-le in that it always denotes the completion of event. It is further argued that the reason why le conveys distinct meanings lies in its position in the sentence. And thus, the conclusion is drawn as: although verb-le and sentence-le denote different meanings, they are essentially the same in terms of the three following aspects: (1) Both of them are perfective aspect viewpoint markers. (2) They have the same temporal interpretations. (3) They show the same variance from guo. Therefore, there is just one le which is a perfective aspect marker. If it is placed after the verb, it will have the terminative reading. If it is in the sentence-final position, it will form a Perfect and have a change-of-state reading. Given such difference, a syntactic representation of le in the clausal structure is proposed, in which verb-le is generated in the AspP lower within vP while sentence-le resides in the AspP adjoined to TP. The reason why guo cannot occur with verb-le is the different aspectual meanings they convey. Guo is concerned with the experience the subject has while sentence-le is about the state change.