The purpose of this study is to measure the text coherence in order to evlauate the text difficulty. The text coherence is deeper level of language variable that have a influence on the text difficulty. But it has been neglected in the assessment of the readability due to the difficulty of measurement. Therefore, the study introduced Latent Semantic Analysis (LSA) as a alternative technique to measure the coherence. And then, 14 Informational texts of Korean Language Textbook were anlayzed. The results showed informational texts of Korean Language Textbook were needed to revise on the text difficulty.