This study was designed to identify the phytochemicals of Allium monanthum with antiproliferative effect on HT-29 human colorectal adenocarcinoma cells through the activity-guided fractionation and isolation method. In particular, compounds 1-8 were isolated from A. monanthum, and determined to be apigenin (1), isorhamnetin (2), 5,7,3′,4′-tetrahydroxy-3-methoxyflavone (3), luteolin (4), myricetin (5), isorhamnetin-3-O-β-D-glucoside (6), isorhamnetin-3-O-β-D-rutinoside (7), and luteolin-7-O-β-D-glucoside (8) by comparing the 1D-, 2D-NMR, and ESI-MS spectral data. Of these, compound 5 significantly decreased the cell viability (IC50, 48.21 ± 1.07 μM) with a morphological change, indicative of apoptosis, resulting in activation of caspase-9, caspase-3, and poly-ADP-ribose polymerase in a dose-dependent manner.