A new photosensitizer Hematoporphyrin Oligomers (HpO) based photodynamic therapy (PDT) on mouse squamous cell carcinoma was studied.<BR>Morphological changes of the tumor after PDT was investigated by a transmission electron microscopy. One hour after PDT, bullous mitochondria were seen with irregular shaped inner cristae. After six hours, the inner cristae and outer membrane of mitochondria disappeared. Endonucleus vesicles appeared. Heterochromatin increased in the nucleus. Forty-eight hours later, mitochondria had no longer its original feature. The outer layer of plasma membrane vanished away.<BR>Our study concludes that target organ HpO based PDT seems to be mitochondria.