Perceptual span in reading, the spatial extent for effective information extraction during a single fixation, provides a critical foundation to all studies for sentence reading. However, it is not understood fully how the perceptual span is influenced by direction-specific reading experience. Traditional Chinese sentences can be written horizontally from left to right or vertically downward, offering the best opportunity to explore readers’ perceptual span in different text directions, free of possible confounding with language proficiency and cross-participant differences. Using a within-item and within-subject design, eye movements of traditional Chinese readers were recorded during their reading of horizontally and vertically presented sentences. Additionally, regardless of text direction, a gaze-contingent moving-window technique was adopted to restrict visible texts within a virtual window that moved in synchrony with the reader’s eye gaze, while characters outside the window were masked. Among several critical results, most importantly, asymptotic reading performance was observed in a smaller window condition for vertical reading than for horizontal reading, suggesting an overall smaller perceptual span in the former case. In addition, the size of the vertical perceptual span increased as a function of the readers’ familiarity with vertical text. We conclude that factors beyond orthographic complexity and readers’ language proficiency can influence the way in which humans read. Readers’ direction-specific perceptual experiences can influence their perceptual span.