Tā rī kh-e Bayhaqī is among the masterpieces of Persian prose, and every Persian-speaking scholar knows about it since it carries both literary and historical merits. The prose language of Tā rī kh-e Bayhaqī is so rich and arresting that makes it unique in the literary history of Iran. texts ever existed in Iran’ s literature history. Bayhaqī is among those writers who have shown their artistry in using Persian words. Since the main focus of this article has been investigating the originally Persian words and terms in Tā rī kh-e Bayhaqī , the linguistic features of pure Persian language in his prose have been analyzed and studied via a descriptive-analytical approach. To do so, the derivation and word formation of the prose has been explored first, and then the etymology and history of different parts of speech have been traced back, and as the last step, the frequency of the words in the book has been presented quantitatively. In addition to Arabic, Turkish and Mongolic words, Bayhaqī has used Pahlavi, Avestan and Persian words abundantly in his work. The most frequently-used Persian words in the book are nouns, verbs, adjectives and adverbs respectively, and they are either simple words, derived nominal or compounds structurally; though derived nominal and compound words are more frequented than simple words.