Textbooks are one of the primary sources for students to obtain knowledge, so they should present accurate knowledge through textual and visual representations. The goal of the current study is to examine the representations in middle school science textbooks based on the diagram coding scheme to find out a general picture of how representations used in the science textbooks over the fifteen years. The sample consists of 6247 representations from twelve middle school science textbooks (four each of sixth, seventh, and eighth grades) from 2002 to 2017. Content analysis was used to analyze the representations in textbooks, which were gathered by document analysis. The representations were evaluated concerning the combination of two main diagram coding schemes. Findings showed that iconic representations are prevalent in middle school science textbooks. There are limited charts, graphs and augmented reality representations in the science textbooks. Furthermore, there are more male representations than female ones, representations are mostly indexed in the main texts, and captions are mainly problematic in middle school science textbooks. The findings based on the two diagrams coding scheme are mainly coherent with each other. Science textbooks should encourage students to interpret and translate between different representations to enable them accurate knowledge.