This article is a corpus based study of restrictive relative clauses in Classical and Modern written New Persian from the 10th to the 20th century A.D. Different types of restrictive relative clauses (indefinite and specific) are described for the marking of the antecedent, either with a demonstrative article or with the indefinite suffix -i or a combination of the two markers. The grammaticalisation process of the -i as a marker of antecedents of restrictive relative clauses is presented in a quantitative study of all occurrences of restrictive relative clauses in the corpus.