Prepositions, in Arabic traditional grammar literature, have been analyzed as Genitive Case assigners (Hasan, 1976; Sibaweihi, n.d.). This paper presents a phase-based analysis for prepositions (Ps) in Modern Standard Arabic (MSA). The analysis is built on Chomsky's (2005, 2008) Feature-Inheritance model of Agree. In this proposed analysis, Prepositional Phrases (PPs) in MSA are analyzed as phases, where a Probe-Goal relation is established between the prepositional Probe p-P and the DP in its searching domain (i.e., its complement). The outcome of this relation is valuation of the unvalued Case feature on this DP complement (i.e., Genitive Case), and a similar valuation to the unvalued phi-features (φ-fs) on the Probe p-P.