Background: Quercetin (QCT) is a dietary flavonoid with many beneficial effects (e.g., antioxidant, antiag-ing, antidiabetic, antifungal effects, and regulation of gastrointestinal motor activity in human); furthermore, it induces apoptosis, cell cycle arrest, and differentiation. Objective: The apoptotic effects of OCT were investigated on SW480 human colon cancer cell lines in monolayer and spheroid cultures. Methods: Quercetin (40-200 1.1M) was applied, and Inhibitory Concentration (IC50) doses were determined for three time intervals (24, 48, and 72 h). The effective dose was determined and applied for analyses, including staining with BrdU to investigate cell proliferation, terminal deoxynucleotidyl transferase dUTP nick and labeling (TUNEL) to in-vestigate apoptosis, and caspase-3 and Apoptosis Inducing Factor (AIF) to investigate caspase-dependent or independ-ent apoptotic pathways. Results: The effective dose of QCT was determined to be 200 1.1M and was found to induce apoptosis and inhibit cell proliferation at 24, 48, and 72 h,both in 2D and 3D cultures. Significant increases were observed in both caspase-3 and AIF staining, but cells showed greater caspase-3 staining compared with AIF staining at all time intervals (p<0.05). Conclusion: The QCT treatment groups showed more cell death and less cell growth compared with the untreated control groups in both 2D and 3D cultures of SW480 cell lines. The results suggest that quercetin induces apoptosis, inhibits cell proliferation, and has a protective role against colon cancer. However, further studies are needed to clarify its mechanism of action.