The expansion of cotyledon pavement cells is regulated by light. However, it is not known how light regulates the expansion of cotyledon pavement cells. Here, we show phytochromes inhibit the longitudinal expansion of pavement cells in response to red light, whereas PHYTOCHROME INTERACTING FACTORs (PIFs) promote the longitudinal expansion of pavement cells. Moreover, supplementation of auxin analogue partially induced longitudinal expansion of pifQ pavement cell. This suggests that phytochromes inhibit the longitudinal expansion of pavement cell by modulating auxin signaling. Moreover, we found that the mutation in LONGIFOLIA1/2 (LNG1/2) reduces cell longitudinal expansion in the dark, while its expression is both induced by PIF and auxin. Together we revealed that phytochromes inhibit the longitudinal expansion of pavement cells by repressing the expression of LNG1/2 through PIF and auxin signaling.