In this research, the photocatalytic degradation of 4-chlorophenol (4-CP) in TiO2 aqueous suspension was studied. TiO2 photocatalysts were prepared by sol-gel method. The dominant anatase-structure on TiO2 particles was observed after calcining the TiO2 gel at 500 ℃ for 1 hr. Photocatalysts with various transition metals (Fe, Cu, Nd, Pd and Pt) loading were tested to evaluate the effect of transition metal impurities on photodegradation. The photocatalytic degradation in most cases follows first-order kinetics. The maximum photodegradation efficiency was obtained with TiO2 dosage of 0.4 g/L, retention time of 1 min and air flow rate of 2,500 cc/min. The photodegradation efficiency with Pt-TiO2 or Pd-TiO2 is higher than pure TiO2 powder. The optimal content value of Pt and Pd is 2 wt%. However, the photodegradation efficiency with Fe(1.0 wt%)-TiO2 and Cu(1.0 wt%)-TiO2 is lower than pure TiO2 powder.