Conditional sentence is a compound sentence consisting of two clauses: protasis and apodosis. The purpose of this paper is investigating the conditional sentences in contemporary Persian language based on the Declerck and Reed typology (2001). According to this semantic typology, possible world of protasis is divided in to two factual and theoretical classes. Theoretical possible world can be neutral or nonneutral and the nonneutral is divided in to four types: closed, open, tentative and counterfactual. In the present study, we try to examine these types with respect to the corpus of 286 conditional sentences extracted from ten contemporary story books and the frequency of occurrence of each one to be determined separately. The analysis of Persian conditional sentences shows that the Persian conditional construction is compatible with mentioned typology and open possible world has the highest occurrence frequency, and the neutral, closed, factual, tentative and counterfactual possible worlds respectively dedicated the highest frequency of the open possible world. The higher frequency of occurrence of the open possible world arises from the fact that in this possible world a probabilistic situation is considered and there is no certainty about its occurrence and this interpretation is most consistent with the operation of the conditional construction. This study also shows that in addition to the common conditional conjunctions such as “ agar, be shartike, vaella, vagarna, ...” other conjunctions are also used as conditional conjunctions like : “ valo, farzan, hamin ke, be mojarade inke, vaghti, cheo…che, hala ke, ta, ke ….” .