In the present study, the frequency effect was examined specifically in text reading in developing and expert readers. Third graders, fifth graders, and expert readers read the same short text containing high- and low-frequency words while their eye movements were recorded. The frequency effect was observed in temporal and spatial measures of eye movement in all participants. However, we found a greater frequency effect in developing readers than in expert readers, and also in 3rd graders than in 5th graders. Furthermore, the frequency effect emerged at an early stage, as soon as the first fixation duration in 3rd graders and gaze duration in 5th graders. In contrast, the frequency effect was observed only for total fixation in expert readers. We discuss these results within the framework of the computational model E-Z Reader to explain why the magnitude of the frequency effect could be different across reading levels.