Grammaticalization provides a framework to study the genesis and development of grammatical forms in language. In this framework, it is argued that new grammatical forms are constantly developed from lexical items and the already formed grammatical items may go out of use. In spite of some counterexamples, this is often seen as a unidirectional and cyclic process. In this article the concepts of cycle and unidirectionality are touched upon. An emphasis is put on the nature of semantic change in grammaticalization. This is a point where grammaticalization and some frameworks of cognitive linguistics converge. In this connection, the framework of metaphorical extension is elaborated on. On the basis of this latter framework, some instances of grammaticalization in New Persian are touched upon. Our evidence shows that metaphorical extension plays a crucial role in the process of grammaticalization. The change from concrete to abstract meaning in the process of grammaticalization, as seen in metaphorical extension, is in line with the concept of unidirectionality. This kind of semantic change is almost without exception in New Persian.