French beginning readers might rely on syllables during reading acquisition. However, no in-depth developmental study has been carried out to determine how and when the syllable becomes this prelexical and segmental unit used in the time course of reading acquisition. We recruited 800 French-speaking children distributed in grade 1-5. We used a lexical decision task in a visual masked priming paradigm and a visual identification task. We manipulated the initial syllable frequency, the initial bi/trigram frequency, and the initial syllable structure (CV; CVC). Our main results describe a clear developmental course. The syllable-based effects are early (G1) and sustainably observed (G5), and primarily depend on the syllable frequency. From G2, we found the systematic, automatic use of the syllable as prelexical and segmental unit but the syllable frequency has facilitatory syllable-based effects in the task with lexical access, while it has inhibitory effects in the task without lexical access.