Photodynamic therapy (PDT) is a novel therapeutic approach for combating various cancers. PDT involves the administration of a photosensitizer which generates singlet oxygen after light activation. FosPeg^Ⓡ is the liposomal formulation of mTHPC. In this in vitro study, the photodynamic efficacy of FosPeg^Ⓡ on a human colon cancer cell line (HT29) was investigated via studying the cellular uptake of FosPeg^Ⓡ, FosPeg^Ⓡ PDT mediated photocytotoxicity and the cell death mechanism were triggered. FosPeg^Ⓡ PDT demonstrated its antitumor effect in a drug and light dose-dependent manner in HT-29 cells. Lethal dose (LD₅₀) was achieved with 0.4 μg/mL of drug and 3 J/cm^-2 of light dose. FosPeg^Ⓡ PDT triggered apoptotic cell death via activating caspase cascade and regulating cell cycle progression. In conclusion, FosPeg^Ⓡ-PDT is an effective measure to combat human colon cancer cells.