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. FosPegr is the liposomal formulation of mTHPC. In this in vitro study, the photodynamic efficacy of FosPegr on a human colon cancer cell line (HT29) was investigated via studying the cellular uptake of FosPegr , FosPegr PDT mediated photocytotoxicity and the cell death mechanism were triggered. FosPegr 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. FosPegr PDT triggered apoptotic cell death via activating caspase cascade and regulating cell cycle progression. In conclusion, FosPegr-PDT is an effective measure to combat human colon cancer cells.