Leaf morphogenesis relies on adaxial/abaxial patterning and extensive growth. This study investigated the role of ANGUSTIFOLIA3 (AN3) from Arabidopsis thaliana in these processes. The an3 mutants produce narrower leaves that contain significantly fewer cells than the wild type. We examined the genetic interaction between an3 and asymmetric leaves2 (as2), which has a weak adaxial defect. The an3 as2 mutants developed trumpet-like leaves and accumulated transcripts of abaxially expressed genes at higher levels than an3 and as2. Gene expression analyses suggested that an3 altered the expression of a number of genes. Many of them were involved in metabolism, and several genes that promote adaxial identity cooperatively with as2 were down-regulated. Next, we performed detailed developmental analyses to examine the relationship between the narrow-leaf phenotype of an3 and leaf polarity. As a result, we showed that AN3 is required during a specific phase after an oblong shape is established in early leaf primordia. During this phase, the angle of the cell division plane relative to the longitudinal axis of the leaf primordium is more variable than in the earlier phase where transverse divisions were dominant in both the wild type and an3. Correlated with this dynamic change in cell division pattern, the leaf primordium became rounder. In an3, mitotic activity was reduced more rapidly than in the wild type, causing premature termination of the morphometric change. These results suggest that AN3 promotes cell proliferation during a specific developmental phase that is also required to correct abaxial/adaxial patterning in concert with AS2.