Applying blended materials with finer particle size or high reactivity could be an effective and economic way for improving the microsturcture of interfacial transition zone (ITZ).In this study, the porosity characteristics of ITZ in concrete made with OPC and blended binders were determined quantitatively by using backscattered electron microscopy (BSE) image analysis and mercury intrusion porosimetry (MIP) measurements.This paper especially focused on the effects of slag and limestone filler on the thickness and pore structure of the ITZ.Results indicated that the porosity at each distance reduces with increasing limestone filler from 0 to 5%, and a significant increase is observed in the sample with 10% of limestone filler.The addition of 5% of limestone filler is able to densify the pore structure of both ITZ and bulk matrix.The reduction in pore volume in the range coarser than 100 nm contributed to the largest decrease in the total pores.Increasing the incorporation level of limestone filler to 10% resulted in an increase in the total porosity.The influences of slag on the porosity characteristics were highly dependent on the replacement level and the determined pore size regions.The addition of 35% of slag reduces the porosity at all distances and produces a denser microstructure both in the ITZ and bulk cement matrix.However, this improvement disappears when the substitution amount reaches to 70%.The incorporation of slag as a partial substitute for Portland cement tends to refine the pore structure.