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In pyrolysis of biomass pellet, heterogeneity in char is almost inevitable, due to the intra-particle mass/heat transfer limitation. In this study, the effects of temperature (350–600 °C) and aspect ratio of a cylindrical sawdust pellet (Diameter = 6, 8, 12, or 16 mm) on the heterogeneity of char structure at radial and axial directions were investigated with a micro-Raman spectroscopy. Based on the spatial distribution of Raman band area ratios, heterogeneity index was proposed to quantify the degree of heterogeneity in char structures. Results show that the heterogeneity exists in both radial and axial directions and could be weakened by the increase of temperature or enlarging the variance between radius and height of the pellet through enhancing the average heat and mass transfer inside the pellet. The heterogeneity index of different biochar decreased with increasing surface-area-to-volume ratio. The char structures was the most heterogeneous (HI(A D /A G) = 5.399 × 10−5, HI(A D /A (Gr+Vr+Vl)) = 2.359 × 10−5) for the pellet with a diameter of 8 mm. • Effects of temperature and aspect ratio on heterogeneity in char were investigated. • The heterogeneity of char structure at radial and axial directions was quantified. • A heterogeneity index was proposed to reflect heterogeneous structure in char. • The heterogeneity of pellet char could be weakened by the increase of temperature. [ABSTRACT FROM AUTHOR]