Colorectal cancer (CRC) is an aggressive malignancy. Critical mechanisms that support CRC progression include cell migration, invasion, metastasis, and angiogenesis, which is associated with L1 cell adhesion molecule (L1CAM) and nuclear factor-kappa B (NF-κB) signaling pathways. In this study, viability of HT-29 cells and human umbilical vein endothelial cells (HUVECs) was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays, and cell apoptosis was investigated by flow cytometry assays. HT-29 cell migration and invasionwere observed by wound healing and Transwell invasion assays, respectively, and tube formation of HUVECs was observed by tubulogenesis assays. L1CAM and NF-κB protein expressions in HT-29 cells treated with onion peel extract were determined by indirect immunofluorescence. Results showed that highdose treatments of onion peel extract inhibited cell viability of both HT-29 cells and HUVECs, induced HT-29 cell apoptosis, and inhibited HT-29 cell migration and invasion. Moreover, onion peel extract decreased total HUVEC tube length and, at a concentration of 10 μg/mL, showed potential to downregulate L1CAM and NF-κB. In conclusion, onion peel extract inhibits HT-29 cell growth, migration, and invasion through suppressing pathways related to angiogenesis downstream of L1CAM-activated NF-κB.