Abstract: 【Objective】This work was conducted to elucidate the cellular mechanism underlying the leaf-size reduction of Pseudosasa japonica var. tsutsumiana. 【Method】Microscopic observations of paraffin sections were used to compare the cell morphology. The Gielis formula was used to precisely analyze the morphologies of the leaf blades of P. japonica var. tsutsumiana and P. japonica. 【Result】The width, length and thickness of the P. japonica var. tsutsumiana leaf are significantly smaller than those of P. japonica. However, leaves of both P. japonica var. tsutsumiana and P. japonica could be precisely described by the Gielis formula. The anatomical analysis indicated that most of the leaf cell morphologies of P. japonica var. tsutsumiana are similar to those of P. japonica, except for the bulliform cells, which exhibit an extremely irregular morphology. Cellular analysis discovered that the width and length of the epidermis cells of P. japonica var. tsutsumiana are similar to those of P. japonica, but the cell number of the former is significantly smaller than that of the latter. Additionally, the fiber cells of P. japonica var. tsutsumiana are shorter but wider than those of P. japonica. 【Conclusion】The reduction in the size and number of cells results in the leaf size reduction of P. japonica var. tsutsumiana.