Introduction and goal: Textbooks are among the most fundamental ways of transmitting educational messages. An important characteristic of textbooks is that their content should be commensurate with the cognitive development level of their intended audience. This study aimed to investigate the readability of the Thinking and Research textbook for sixth-grade elementary students using Shannon’s entropy method. The statistical population in this applied study consisted of the content of the Thinking and Research textbook for sixth-grade elementary students.

Research methodology: This study employed content analysis, objective-quantitative methods, and Shannon’s entropy method for data analysis.

Findings: The study findings showed that the texts analyzed by methods proposed by McLaughlin and Gunning Fog were higher than the average abilities and cognition of sixth-grade elementary students. The results also demonstrated that the highest and lowest coefficients of importance in the studied textbook were related to Chapter Four (0.269) and Chapter One (0.233), respectively.

Conclusion: Based on the study findings, it can be concluded that the content of textbooks should be regularly evaluated and measured in order to improve their efficiency and effectiveness and achieve educational goals.