This study aimed to empirically examine the polarization of reading ability by conducting a longitudinal analysis of the developmental patterns in reading ability, tracking the same group of students from the first year(Time 1) to the third year(Time 2) of middle school. To achieve this, we conducted latent class analysis(LCA) and multinomial logistic regression. The analysis revealed that overall reading ability had improved significantly, but the reading ability gap between students actually widened. The development patterns were distinct in factual and inferential understanding across different areas, and notable differences by gender emerged over time. The latent class analysis revealed that reading ability was categorized into three distinct groups: upper, middle, and lower. Notably, the proportion of the middle group decreased, and the gap between the upper and lower groups widened. Furthermore, it was found that reading motivation is a significant predictor of reading ability. Finally, this study suggests the need for continuous and systematic management of reading ability development by tracking its developmental history.