CONSTANS-LIKE (COL) family members are commonly implicated in light signal transduction during early photomorphogenesis. However, some of their functions remain unclear. Here we propose a role for COL13 on the hypocotyl elongation in Arabidopsis thaliana. We found that COL13 RNA accumulates to high levels in hypocotyl, and that disruption of COL13 function via T-DNA insertion or RNAi led to longer hypocotyl of Arabidopsis seedlings in red light. On the contrary, overexpression of COL13 resulted in shorter hypocotyl. With various genetic, genomic and biochemical assays, we proved that another COL protein named COL3 directly bound to the promoter of COL13, and the promoter region of COL3 was targeted by the transcription factor LONG HYPOCOTYL 5 (HY5), to form a HY5-COL3-COL13 regulatory chain for regulating hypocotyl elongation in red light. In addition, further study demonstrated that COL13 interacted with COL3 and COL13 promoted the interaction between COL3 and CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1), suggesting a possible COP1-dependent COL3-COL13 feedback pathway. Our results provides new information regarding the genes network in mediating hypocotyl elongation.