Textile industry is one of the most common and essential sectors in the world. On the other hand, high volume of water consumption and varying wastewater characteristics due to many products such as dyes, biocides, carriers, detergents, etc. used in the process are the factors that have caused a continuous effort to find appropriate technologies to treat textile industry wastewater. In the last decade, the use of advanced oxidation processes (AOPs) has gained importance to degrade, decolorise and detoxicify the textile wastewater. However, the main mechanisms for AOPs for removing toxicity, thus, any contribution by the reduction of decreased inert factions in the effluent of textile wastewater are still to be studied. This study aimed to investigate the relation between inert COD fractions (which are characteristics in part of the influent wastewater, and in part generated in the biological treatment as the common used treatment process for textile industry) and effluent toxicity in the presence/absence of pre-ozonation. 18.5 mg/ I of ozone dose was applied during 30 min oxidation on two wastewater samples taken from balancing tank of a textile industry localized in Istanbul, Turkey. A direct assessment method was used to determine inert COD fractions of the raw and ozonated wastewater samples. The toxicity of raw and treated was tested using newborn (<24 h) samples of Daphnia magna for 24 h exposure time. The soluble inert COD (S) decreased significantly while soluble residual microbial product (S-P) increased slightly due to preozonation. The sum of soluble inert fractions were found in the order of 17-18% and 20% of total COD for raw and preozonated samples. Preozonation decreased the toxicity of raw wastewater. This phenomenon was evidenced also by testing the effluents of the inert COD determination reactors.