Two-dimensional image analysis is usually used to obtain the thickness of the interfacial transition zone (ITZ) around aggregates. However, if the two-dimensional section plane crosses through the ITZ region but not through the corresponding aggregate, this region (called invalid region) will be excluded from the analysis. This exclusion will induce bias in estimating the thickness t of the ITZ. This paper proposes a methodology to quantify the bias on the ITZ thickness induced by areal analysis. The approach includes three parts: (1) Uniform ITZ layers around ovoidal aggregates are generated based on the Minkowski sum; (2) A systematical cap sampling method (SCSM) is proposed to estimate the volume of the invalid region V_ITZiv; (3) By combining with V_ITZiv, the underestimation degree of the ITZ thickness for dilute mono- and multi- packing systems are calculated. The results reveal that the underestimation degree increases with the increasing sphericity of aggregates.