Plants have diversified their leaf morphologies to adapt to diverse ecological niches. The molecular components responsible for regulating leaf morphology, however, have not been fully elucidated. By screening Arabidopsis activation-tagging lines, we identified a dominant mutant, which we designated longifolia1-1D (Ing1-1D). Ing1-1D plants were characterized by long petioles, narrow but extremely long leaf blades with serrated margins, elongated. oral organs, and elongated siliques. The elongated leaves of the mutant were due to increased polar cell elongation rather than increased cell proliferation. Molecular characterization revealed that this phenotype was caused by overexpression of the novel gene LNG1, which was found to have a homolog, LNG2, in Arabidopsis. To further examine the role of the LNG genes, we characterized Ing1 and Ing2 loss-of-function mutant lines. In contrast to the elongated leaves of Ing1-1D plants, the Ing1 and Ing2 mutants showed slightly decreased leaf length. Furthermore, the Ing1-3 Ing2-1 double mutant showed further decreased leaf length associated with less longitudinal polar cell elongation. The leaf widths in Ing1-3 Ing2-1 mutant plants were similar to those in wild type, implying that the role of LNG1 and LNG2 on polar cell elongation is similar to that of ROTUNDIFOLIA3 (ROT3). However, analysis of a Ing1-3 Ing2-1 rot3-1 triple mutant and of a Ing1-1D rot3-1 double mutant indicated that LNG1 and LNG2 promote longitudinal cell elongation independently of ROT3. Taken together, these findings indicate that LNG1 and LNG2 are new components that regulate leaf morphology by positively promoting longitudinal polar cell elongation independently of ROT3 in Arabidopsis.