The effect of ozone oxidation on bio-degradability of leachate was studied. Ozone oxidation process was used as pre-treatment process to enhance performance of biological process in treating landfill leachate. Optimum ozone injection rate and contact time in this experiment was $160{;}mg{;}O_/{ell}{cdot}hr$ and 45 minutes, respectively. Bio-degradability was enhanced 5.08% by ozone oxidation. The ratio of ozone demand/DOC concentration was $0.049~0.091{;}mg{;}O_3/mg{;}DOC$. The increase of bio-degradability depending on ozone injection rate(D) and contact time(T) can be expressed as follows ; <TEX<${Dalta}E{;}={;}0.00479{cdot}D^{0.773}{cdot}T^{0.800}$</TEX> The rate of bio-degradation of DOC was increased proportionally with the increase of ozone injection rate and contact time irrespective of landfill site age. The increase of bio-degradability by ozone addition was not satisfactory. It is hard to expect significant increase in bio-degradability by ozone treatment only. Thus, it is evaluated that ozone oxidation can not increase biodegradability significantly in concentrated wastewater composed of complex organic compound such as leachate.