There are some sorts of `Preposition + Noun' combinations in Farsi that apparently a Prepositional Phrase almost behaves as Compound Prepositions. As they are not completely behaving as compounds, it is doubtful that the process of word formation is a morphological one. The analysis put forward by this paper proposes "incorporation" by which an No is incorporated to a Po constructing a compound preposition. In this way tagging prepositions and parsing texts in Natural Language Processing is defined in a proper manner.