The aim of this work was to study the possibility of modifying three types of whey protein concentrations (WPC) chemically to improve their functional properties and use them to make fortified yogurt. Salted and acid whey protein concentrations were obtained by acid/heat or ultra filtration treatments. It was modified by acetic acid anhydride at the rate of 0.3, 0.5 and 0.7g/g protein. Acetylation of salted whey protein concentrate (SWPC), acid whey protein concentrate (AWPC) and filtrated whey protein concentrate (FWPC) improved its functional properties such as water absorption, emulsification capacity and gelation. The best treatment was acetylated WPC at 0.5g acetic acid anhydride/ g protein. Concerning the type of whey protein concentration FWPC exhibited the best functional properties followed by SWPC and then AWPC. Fortification of cows milk with modified WPC to make yogurt caused a pronounced increase in total solids, ash, pH and curd tension, while decreased syneresis. Also, addition of acetylated FWPC and SWPC up to 2% increased the acceptability of fortified yogurt.