The catalytic photodegradation of diclofenac, which is a non-steroidal anti-inflammatory drug, from synthetic waste water was investigated under a 254 nm ultra-violet light using MgO nanoparticle as a photocatalyst. The kinetics of the catalytic photodegradation of DF was followed in batch system. It was found that DF degrades to form by-products which are in turn degrades further with slower rates to other by-products and /or CO2 and H2O. The reaction was found to be of consecutive type with the observed pseudo first-order rate constants for the overall catalytic photodegradation (kobs1 = 0.1191, 0.0411, 0.0127 and 0.0052 min−1 for the initial concentrations of DF = 10, 20, 50 and 100 mg L−1). 100% degradation efficiency was obtained in 60 min for 10 mg L−1 DF. The pseudo-first order rate constant for the adsorption (kAds) of DF on 0.1 g MgO-NPs was measured as 0.00762 min−1 and the photolysis rate constant (kp) (0.0069 min−1).