With the aim of production of functional sponge cake, soy protein isolate (SPI) was added at 0, 5, 10, 15, 20 and 30% (w/w), and the quality of batter and the cake was studied. Following changes occurred as the quantity of SPI increased; batter consistency and density, cake volume and height increased while cake density decreased. The crust and crumb color became darker and more reddish. Textural properties of the cakes as measured by a texture analyzer revealed that the hardness and gumminess increased while springiness, cohesiveness and chewiness decreased. Determination of the organoleptic characteristics of the cakes revealed that inclusion of more than 20% SPI had negative effects on the taste, color, texture and overall acceptability of the cakes. Based on the results, addition of no more than 20% SPI can be used to increase the nutritional quality of the sponge cake while preserving its quality. Practical application Development of functional foods has a growing market around the world. This study provides information on the development of a functional sponge cake using soy protein isolate (SPI). To get benefits from the SPI in cake production, controlling the level of the SPI is crucial. The appropriate level of the SPI resulting in an acceptable functional sponge cake was 20% while further increase in the SPI resulted in inferior cake quality.