In wastewater plastic recycling processing plant, commonly contain high chemical oxygen demand (COD) concentration. Coagulation-flocculation, adsorption and foam fractionation are the examples of treatment processes that can reduce COD concentration in wastewater. Steel slag can be used as an alternative to remove COD concentration of wastewater adsorption process. Linz-Donawitz (LD) slag can be obtained from steel manufacturing plant. LD slag also contains alkali oxides, porous characteristic, large surface area and contain an easy solid-liquid separation. This research is to study the percentage reduction of COD in wastewater using LD slag as adsorbent. This research will investigate the effect of particle size, dosage amount and contact time between LD slag and COD reduction. From the experiment, the highest percentage COD removal for particle size is 0.2mm, the dosage amount is 6 gram and the contact time is at 60 min. Comparison of COD removal by using coagulation -flocculation, adsorption using activated carbon and adsorption using LD slag processes was done. LD slag can reduce 2% higher of COD compared to activated carbon. LD slag should be invested more in wastewater treatment process.