Readers' prior knowledge is an important factor for comprehending scientific texts. The study used an eye tracker to examine the reading processes of the undergraduates with high (HPK) and low prior knowledge (LPK) while reading a long printed scientific article. The results of measuring the eye movements offered some interesting findings. First, HPK readers are more capable of using multiple representations and know the importance of scientific diagrams for reading comprehension; the results revealed that HPK readers spent significantly longer fixation durations on representational and statistical diagrams and had higher percentages of transitional fixations between text and diagrams than did LPK readers. Second, LPK readers were much more text-driven; the total fixation durations on text was significantly higher for the LPK readers than for the HPK readers, and they also exhibited a tendency to read the diagram captions rather than the diagrams after reading the text. Third, mature readers in both groups would engage in self-regulating their reading strategy to slow down their reading speed for processing more important information. This study overcame technical limitations and recorded readers' eye movements in a real print reading situation which may open the path to assess broader issues in future reading research.