Quantitative image analysis approach was often employed to obtain the apparent interfacial transition zone (ITZ) thickness around aggregate particles. However, the 117 thickness may be overestimated via sectional plane analysis method, and exaggerate the ITZ's contribution to the macro-properties of cementitious composites. Until present, little work was done to derive the quantitative relationship between apparent and actual ITZ thickness, especially on non-spherical particle. This paper has proposed a methodology to construct the ITZ layer around Platonic particles and has employed a systematic line sampling algorithm to estimate the thickness of ITZ, called apparent ITZ thickness. The influence of particle shape unified by sphericity on the estimation of ITZ thickness unified by degree of overestimation is discussed where the degree of overestimation represents the ratio of apparent ITZ thickness to actual ITZ thickness. The particle shape effect is discussed for a single particle, for multi dispersed spherical systems and for Platonic particle systems, respectively. Finally, the unexpected contribution of ITZ thickness overestimation on ITZ volume fraction and diffusivity of concrete are analysed. (C) 2015 Elsevier B.V. All rights reserved.