The anatomical structure of the bamboo stem is characterized by vascular bundles comprising the xylem, phloem, and sclerenchyma fibrous sheaths as well as parenchymatous ground tissue in which the vascular bundles are embedded. The composition of the stem is the main factor influencing the anatomical characteristics of circular bamboo, which shows considerable variation in the radial direction. However, most species of Chimonobambusa have square stems. Here, we tested the hypothesis that circumferential variation exists in the cross-sectional anatomy of this species. We analysed fibre morphology and the cross-sectional structural characteristics of vascular bundles of Chimonobambusa quadrangularis (Fenzi) Makino and their associated circumferential and radial variation in cross-sections. Microscopic observations were conducted to identify, measure, and compare fibre morphology and the structural characteristics of vascular bundles, including both circumferential and radial anatomical variation. Vascular bundles occurred as undifferentiated, semi-differentiated, and open types in the radial direction with no changes in the circumferential direction. The average length, width, and ratio of fibre length to width were 1463.6 μm, 12.3 μm and 119.3 in the corner region, and 1452.7 μm, 12.8 μm, and 111.3 in the side region, and there were significant circumferential and radial differences in length, width, and the ratio of fibre length to width (). The circumferential variation in density of vascular bundles, the ratio of fibre length to width, radial to tangential diameter ratio of vascular bundles, and the proportion of sclerenchyma were greater in the corner regions than the side regions. The variation in fibre width and the proportion of parenchyma were greater in the corner regions than in the side regions. The density of vascular bundles and proportions of sclerenchyma were greater in the outer stem compared to the inner stem, whereas the length, width, and ratio of fibre length to width were greatest in the centre compared to the inner and outer zones. Circumferential variation of the density of vascular bundles, fibre length and fibre width occurred in the central and outer stem zones. These findings confirm that there are significant anatomical variations in both the circumferential and radial directions and provide a scientific basis for the rational use of Chimonobambusa quadrangularis.