The study aims to explore the effect of illustrations on young readers' comprehension of science passages, including how they decode specific symbols such as arrows when viewing illustrations. We investigated reading behaviours and interpretation of arrow symbols of 64 sixth-graders in three illustrated science passages using an eye tracker and by conducting tests and interviews. Results showed no significant difference between the illustrated and text-only groups on reading comprehension, total fixation duration for the text section, and FD/P ratio (the total fixation duration for each paragraph divided by their area [numbers of pixels]) of the paragraphs with spatial structure information. Furthermore, the average duration of the illustrated group's fixation upon illustrations was less than ten seconds. Regarding reading sequence, the illustrated group's referencing behaviour between text and illustrations were limited and had inappropriate timing. Participants could be aware of the semantic roles of the arrows in major categories but had difficulty distinguishing subcategories. Furthermore, their use of the semantic role of 'labelling' was overextended and that of 'vector' was underextended. The words that young readers use most frequently to refer to the subcategories signified by the arrow symbols were identified. The implications of instructions for understanding diagram conventions are discussed.