The consumption of large amounts of fats in the diet is considered as a risk factor for heart disease, which is now the world’s leading cause of death.Because of increasing consumer trend for low fat products, the production of reduced- or low-fat cheeses has significantly increased since 1980. Pizza cheese is often used as a functional ingredient on pizzas, and its ability to stretch when melted has made this cheese the choice for use as a pizza topping. Fat provides a lubricating effect in cheeses during heating. The reduction of fat in Mozzarella cheese adverselyaffects its functionality and quality. Reduced-and low-fat cheese are usually characterized as having poor body and texture, flavor, and functional properties because of high moisture and low fat.Whey is an excellent source of functional proteins that can be used in food industry. Otherwise it is the most polluting by-product in dairy industries. Wheymodification can improving the performance characteristics of whey proteins in food products. In this study, whey protein structuremodified by using thermal treatment(85°C for 30 Min) at different pHs (5, 7, 9) and ionic strength (1-2% Nacl). Turbidity, particle size distribution and insoluble aggregates content of WPC were measured at before and after freezing dryer. RSM design was used to investigate the effect of pH, thermal treatment and ionic strength on physic-chemical properties of whey protein. In the next step, three different size whey protein aggregateswere used a fatmimeticsinlow fat pizza cheese. Modified whey protein concentrate can also be retarded by increasing the water-holding capacity of the cheese protein matrix. In this research. reduction of 50% of fat and used modified whey protein concentrate and emulsions stabilized by hybrids of natural polymers (encapsulated) a fat replacer to improve the quality of low fat pizza cheeses. And the effects of different levels (0.5-1.5%) of modified whey protein at different aggregate sizes and different levels (1-2%) encapsulatedbutteroilof on physic- chemical and Sensory properties of low Fat pizza Cheese was investigated. Result showed,the size of aggregation and pH ofmodified whey protein Have a significant effect on meltability, stretchability and pH of the final product. Also, the percentage of fat replacer has significant effect on thefunctional properties of low fat pizza. The low fat pizza cheese containedtheleastsizeofmodified whey protei had most desirable functional properties. Keywords Modified whey protein, Low fat pizza, Functional properties, Encapsulatedbutter oil.