Reading strategy instruction has been an important area in educational psychology for decades, however, research has primarily focused on its influence on learning outcomes rather than learning processes; reading pure texts rather than illustrated texts; and immediate effect rather than retention effect. This study used an eye-tracker to investigate the immediate and delayed effects of text-diagram reading instruction on reading comprehension and learning processes in illustrated text reading. Fourth-grade students with high (N = 66) and low reading ability (N = 66) were randomly assigned to one of three groups: a text-diagram group who received text-diagram instruction which emphasized diagram decoding and integration of relevant textual and pictorial information, a placebo group who received instruction which emphasized comprehension monitoring, and a control group which received no reading instruction. All participants read four illustrated science texts for a baseline check, instructional example, immediate testing, and delayed testing. The results showed that the effect of text-diagram instruction was more evident in the immediate test than the delayed test. The eye-movement pattern showed that the students who received text-diagram reading instruction spent significantly more reading time on illustrations, made more integrative transitions between text and illustrations, and spent a higher proportion of total reading time on illustrations in immediate and delayed reading situations than the other groups. Overall, this study developed an effective text-diagram instruction method to promote reading comprehension, identified the reading processes underlying the effect of text-diagram strategy instruction, and depicted the changing appearances of reading instruction intervention over time.