Effect of konjac flour (0.5 and 1%) incorporated with soy protein isolate (SPI) (5 and 10%) against wheat flour on physical,chemical and sensory properties of reducedfatchiffon cakes prepared with 50% vegetable oil replacement with water were investigated. The addition of konjac flour and SPI produced reducedfat cakes with significantly lower (p < 0.05) in specific volume, but higher (p < 0.05) in weight loss and water activity. All reducedfat formulations were significantly darker (p < 0.05)than the control. The high concentration of SPI produced the cakes with higher L*(lightness) but lower a*(red) and b*(yellow).The addition of konjac flour incorporated with SPI significantly decreased (p < 0.05) hardness, cohesiveness and springiness of reducedfat cakes. Sensory results indicated that the cakes with konjac flour and SPI tended to be darker, juicy, sweet and flavour, but less hard than the control; nevertheless, the cake with 0.5% konjac flour and 10% SPI was rated as tender as the control. Also, the reduction of fat and total caloric value of this cake were about 37.9 and 14.6%, respectively, whereas the increment of protein was 24.5% in relation to 100 g of the control cake.