Steel making slag is one of the most important by-products of steel making industrywhich have been used for different purposes because of existence of high content of metals such as types of iron oxide. One of the uses of steel making slag is removing pollutions of industrial wastewater. Nowadays,importance of wastewater treatment activities and water recycling have increased because oflack of suitable natural fresh water resources. Advanced oxidation processes are chemical based methods which are able to remove toxic pollutions and also convert non-biodegradable pollutions to biodegradable type. Fenton process is advanced oxidation methods with a lot of advantages, including low cost, requiring simple laboratory logistics, and environmental friendly with a little demand for catalysts. This study was conducted to investigate the effect of fenton and microwave-fenton processes in removing organic load of industrial wastewaters. Experimental conditions such as pH (2, 3, 4, 5, 6 and 7), hydrogen peroxide (100, 250, 400, 500 ppm), steel making slag (12.5, 25 and 37.5 g/L) and measurement time (0.5, 1, 2, 3 and 4 hour) have been used in order to optimizethe efficiency of heterogenous fenton process in decreasing the chemical oxygion demand (COD). After adjustment of optimized condition, different factors were investigated in this study, including different pH conditions, different concentrations of COD (100, 200, 400 and 700 ppm), EC and Fe 2+ and exposure time (5, 10, 15, 25 and 40 min) and microwave exposure power (100, 300, 450 and 800 watt). Results were analyzed by SAS software with one-way ANOVA algorithm based on least significant difference test, and results of statistic analysis show that COD removal efficiency was about 63.7%in optimized experimental conditions consist of exposure time (2 hour), pH (3), steel slag concentration (25 g/L) and hydrogen peroxide (250 ppm). In addition, pH increased up to 10 and EC decreased from 3.4 to 2.4 Ms/cm in the optimized conditions. The highest removal efficiency for COD was observed at 100 mg/L COD concentration.Microwave exposure power affect the process just with decreasing the time, and the highest efficiency of COD removal was 57.5% at 450 watt of microwave exposure power and 25 minutes exposure time. Therefore,microwave exposure time just affect the required time for COD removing process and there was no obvious effect on removing efficiency. Finally, we conclude thatfenton process is usefullin COD removing of industrial wastewaters because of its acceptable costs, practical method and considerable removing efficiency. Keywords: COD, heterogeneous fenton process, industrial wastewater treatment, microwave exposure, steel making slag