The purpose of this study was to seek strengths and weaknesses from analyzing Korea and U.S. science textbooks in terms of general structural features, contents, cognitive levels of questions and the purpose of questions used in science textbooks. This provided insight into improvement of textbooks that can effectively assist teaching and learning. To investigate organization of unit in textbooks in-depth, the evolution unit was selected and scrutinized as one example. The results showed that the number of pages, activities, vocabulary words, and vocabulary lists are considerably different between Korean and the U.S. Commonly, U.S. textbooks were more laden with information and lacking in coherence than those of the Korean textbooks. The findings on the cognitive levels of questions showed that the majority of questions in both nations are concerned with knowledge. However, the difference between the two nations is great in the ratios of analysis, synthesis, and evaluation questions. Questions are concentrated in review section (45% of Korean and 60.6% of U.S.) in textbooks. It suggested that well-planned questions in a review section can provide the basic guidance for strength in a science classroom.