Interfacial transition zone (ITZ) between matrix and aggregate often plays a major role in transport and mechanical behaviors of cementitious composites as its physical nature of relatively high porosity and low rigidity. However, the ITZ thickness may be overestimated via sectional plane analysis technologies, this is attributed to a random sampling plane rarely passing through the normal to the surface of aggregates. So, it is necessary to quantify the overestimation degree of the ITZ thickness. In this study, a methodology to construct the ITZ layer around ellipsoidal aggregate particles is proposed. Then, a systematic line sampling algorithm is employed to obtain the statistical mean value of the ratio of the apparent to the actual ITZ thickness. The influence of aggregate shape and particle size distribution is discussed on the overestimations of the thickness and volume fraction of the ITZ, and the effective diffusivity of three-phase composites using the differential effective medium approximation. Results show that the aggregate shape has a significant impact on the apparent ITZ thickness, which mainly reflects in the sphericity of the aggregate shape. And the ranking of the overestimation of ITZ thickness, ITZ volume fraction and diffusivity for two types of spheroidal aggregate which have the same sphericity with the specified Platonic particle follow: Platonic particle > Oblate ellipsoid > Prolate ellipsoid. Sphericity is not a unique parameter characterizing the influence of particle shape on the overestimation degree of ITZ thickness. For quantification, other shape parameter needs to be developed in the future. We hope this article will be helpful to guide the practice of evaluating the contribution of aggregate particle shape to the ITZ microstructure and the influence of the 177 on macroscopic performance of composite materials. (C) 2017 Elsevier B.V. All rights reserved.