Machine translation of English sentences faces a big problem when it deals with phrasal verbs. Phrasal verb is a common structure occurring in English as a combination of a verb and a preposition, a verb and an adverb, or a verb with both an adverb and a preposition. Meaning of a phrasal verb is not compositional. The second part of the phrasal verbs which often is a preposition is called particle. The process of detecting a preposition as a particle or as an attachment in a preposition phrase can be a challenging problem. In this paper, we present a method which uses a combination of linguistic heuristic rules with a probabilistic English parser to disambiguate the role of prepositions. The aim of this disambiguation is to correctly detect the phrasal verbs in English to Persian machine translation system. Experiments on a corpus containing 520 sentences show that the quality of phrasal verb recognition in this system grows up to 87%.