Leaf morphogenesis relies on continuous cell proliferation and specification of distinct types of cells. During early leaf development, leaf cells mainly undergo vigorous mitotic cell divisions to boost the number of primordial cells. Along leaf growth, leaf cells gradually exit from the mitotic cell cycle from leaf tip to base and enter the endoreduplication, which is frequently associated with cell expansion and differentiation, resulting in cells with increased nuclear ploidy levels. Here we describe the methods and several examples employing flow cytometry to investigate the cell cycle regulation during leaf development in Arabidopsis thaliana, a model dicotyledonous plant. We found that there are at least three types of leaf developmental mutants with abnormal cell cycle. In addition, we introduce the assay to measure the efficiency of DNA replication in Arabidopsis.