Abstract: The objective of the present study is degradation study of 4-chlorophenol (4-CP) as highly toxic pollutant by a new designed Pt/γ-Al2O3 nanocatalyst. The proposed shaped controlled nanocatalyst was synthesised in microemulsion systems containing 25% or 0% HCl. The prepared nanocatalysts were characterized by several techniques such as scanning electron microscopy (SEM), cyclic voltammetry (CV) and X-ray diffraction (XRD). The effect of some important parameters such as initial concentration of 4-CP (10-50 mg/L), solution pH (3-11), and temperature (20-40 0C) on the degradation process was investigated by Box–Behnken design. The obtained results showed that HCl concentration of the microemulsion system can affect the catalytic activity of Pt/γ-Al2O3. It is confirmed that the catalyst synthesized in microemulsion system containing 25% HCl is most effective and maximum degradation was obtained at pH=11, [4-CP]0=50 mg/L and reaction temperature of 400C.