Effects of the capillary number and size distribution of bubbles on rheological properties of bubble suspensions with different volume fractions were investigated through both experimental and theoretical analyses. Polydispersed bubble suspensions were prepared by using a bubble making device and characterized in steady and dynamic oscillatory shear flows to examine the effect of the bubble size distribution. To predict the rheological properties of the polydispersed bubble suspension, a constitutive equation was modified by applying superposition schemes, i.e., a probability density function was introduced to reflect the size distribution of bubbles. Dilute bubble suspensions showed shear thinning and linear viscoelastic behavior. At small capillary numbers, relative viscosity increased with increasing bubble volume fraction, whereas it decreased as bubble volume fraction increased at relatively large capillary numbers. It was found that rheological properties had been affected significantly by polydispersity of bubble suspensions and theoretical results were in good agreement with the experimental results. POLYM. ENG. SCI., 50:128-137, 2010. (C) 2009 Society of Plastics Engineers