The purpose of this study is to analyze and compare the similarity and the difference between English free relative clauses and indirect question clauses based on High school textbooks. Through this process, after surveying syntactic and semantic characteristics of English free relative clauses and indirect question clauses, this study finds the way to identify two things. First, because the syntactic free relative clause which includes ‘what’ has characteristics of modifier, extraposition is impossible. It is on a theoretical basis to separate between ‘what-free relative clause’ and ‘what-indirect question clauses’. Second, free relative clauses include the antecedent. Therefore, it is impossible to move preposition in front of the sentence in a free relative clause. However, indirect question clauses can move preposition forward ‘what- interrogative.'' Lastly, according to syntactic and semantic characteristics of the main verb, free relative clause and indirect question clause can be distinguished. As mentioned earlier, If meaning contained in the main verb describes the subordinate content or it explains the situation, it can be a free relative clause. If the verb of the main clause isn’t definite like doubt, supposition, assumption, etc., it can be indirect question clause. To explain again, in the free relative clause, the speaker knows already the content. Indirect question clause shows a speaker suggest that they do not know the content. Based on the theoretical reasons, this thesis compares free relative clause with indirect question clause in the body of 1st grade High school English Textbook. It is important for the students to find out differences between a free relative clause and an indirect question clause. Based on these results of the study, teachers can fully understand syntactic and semantic similarity and difference of the free relative clause and indirect question clause. So teachers can give students an explanation that is easier to understand. As a result, it is expected that both teachers and students to learn English can get some study courage with confidence.