Porous TiO 2 nanocrystals (PTN) were synthesized using activated carbon templates with supercritical CO 2 by using the nanoscale permeation (NP) process. The photoactivity of PTN was tested by methylene blue (MB) degradation. Compared with the commercially available P-25, all PTN exhibited significant photocatalytic degradation of MB mainly due to their porous structure with high surface area, high hydroxy concentration and small crystalline size. The optimum temperature and pressure are found to be 60°C and 26MPa, under which obtained PTN-1 shows the highest photoactivity and slow deactivation for MB degradation after 15 trials.