Mineralization of circumpulpal dentin has been interpreted in such a way that predentin matrix is abruptly converted to almost fully mineralized dentin at the mineralization front. A group of investigators pointed out the existence of intermediary layer along the mineralization front of rat incisor dentin and claimed that dentin mineralization is a rather transient process. Owing to a paucity of information, however, the entity of transient mineralization of dentin has remained elusive. Here we confirmed the existence of a lightly mineralized layer (LL) along the mineralization front of rat incisor dentin, recognizable by both light and electron microscopy, in routinely processed specimens. LL less than 3 µm thick was shown to be located along the mineralization front of crown-analog dentin and tapered out toward the root analog of the incisor. Electron microscopy revealed that mineral deposition first occurred in the non-collagenous matrix of LL and that mineralization of collagen fibers took place sometime later at the conventional mineralization front. Microscopic appearance of the mineral phase of LL varied considerably depending on the histological processing of ultrathin sections, thus explaining the inconsistent interpretation of dentin mineralization in previous studies. These data suggest that mineralization of circumpulpal dentin in rat incisors proceeds in a stepwise or a transient manner, initiated by crystal deposition in the non-collagenous matrix followed by massive mineral deposition in collagen fibers at the mineralization front. The thickness of LL where only the non-collagenous matrix is mineralized may vary in relation to differences in the local non-collagenous matrix and also the rate of collagen mineralization in the respective portions of circumpulpal dentin.