Using the language-specific behavior in information retrieval systems can improve the quality of the retrieved results significantly. Part of the word that remains after removing its affixes is called stem. Stemming process can be used for improving the relevancy of the results in information retrieval system. Different morphological variants of words (plural, past tense…) will be mapped into their stem which can be used in the searching process of information retrieval tasks. Using the stem instead of the surface of the word reduces the size of the index file significantly. In this paper, an algorithm for stemming Persian words is described and its effect on information retrieval system is evaluated with different ranking methods. By using Persian Porter stemmer with just 43 rules, the size of index file reduced about 5% while the mean average precision of the retrieval information system improved about 5%.