Effect of butter content (0, 40, 80 and 120% to flour) on the physical properties and the structure of sponge cake batter (200g of egg, 100g of sugar, 100g of flour and O.5g of baking powder) was studied by using a rheometer and a cryo-scanning electron microscope (SEM). The fluidity of batters increased remakably with in creasing butter content from 0 to 120%, and that of batters aged for 10 to 30 min increased with increasing butter content up to 80% and decreased slightly by the addition of 120% of butter. Both dynamic modulus (G') and dynamic loss (G") of batters aged for 10 to 30 min decreased slightly with increasing butter content up to 80%, and the values of batters increased slightly by the addition of over 80% of butter. Significant negative correlation was obtained between the dynamic modulus and the fluidity of batters and also between the dynamic loss and the fluidity of batters. The loss tangent (tanδ) of batters aged for 10 to 30 min decreased with increasing butter content. The change in tanδ with butter content was similar to the change in parameter C obtained from the formula (y=An^Ce^Bn8) derived by 10 biting trials reported previously (y=overall hardness, B=rate of breakdown, C=resistance to breakdown). In addition, the significant correlation was obtained between the tanδ and specific cake volume. Thus, the tanδ seems to be an important parameter explaining the physical properties of sponge cake batter. Mixture of large and small round air cells were observed in the batters with butter content of 0, 80 and 120% by SEM observation. Plication was observed clearly at the surface of lipid globules of 20-60/μm in diameter. From these results, it is concluded that the best physical properties and structure of sponge cake batter are obtained by addition of 80% of butterto flour.