In this paper, the photodamage of Hypocrellin A to the fluidity of human erythrocyte membranes and some kinds of membranes of phospholipid liposomes was investigated by measuring the changes in fluorescence polarization of the membranes. The results showed that the photosensitization effect of HA caused the decrease of membrane fluidity of the phospholipid (DPPC, DPPC/DPPE, phospholipid of erythrocyte membranes) liposomes. The DPPC and DPPC/DPPE liposomes were more sensitive to the damage than the phospholipid liposomes of erythrocyte membranes. To human erythrocyte membranes, the photodamage effect of HA caused its fluidity first increased and then, with the increment of illumination time, decreased. To spectrin-depleted and trypsin-treated erythrocyte membranes, this kind of change in fluidity was inhibited. All of the results indicated that phospholipids and proteins play different roles in the photodamage of HA to the fluidity of membranes. Membrane proteins, especially spectrin, were the key factor involved in the changes of the fluidity.