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Graphite and graphene have attracted tremendous research interests in the past decade because of their superior electrical and thermal properties.So far,extensive studies have been conducted on thermal transport properties of graphite and graphene,especially on in-plane thermal conductivity [1-3] and contact thermal resistance between graphene and other materials [4].However,the phonon mean free path of graphite in the cross-plane direction has not been well understood yet.For example,the simplified kinetic theory predicted that the cross-plane phonon mean free path of graphite is only a few nanometers [5],but recent molecular dynamics simulations indicated that it could be as large as 2000 nm at room temperature [6].In this work,we experimentally tackle this problem by carefully characterizing the cross-plane thermal conductivities of graphite flakes with thicknesses ranging from tens to hundreds nanometers via the three omega method.The graphite flakes are prepared on the silicon substrate by the mechanical exfoliation method.At 300 K,the measured cross-plane thermal conductivities of graphite flakes with thicknesses ranging from 20 nm to 400 nm are substantially lower than the counterpart of the bulk graphite and increase with the thickness.Our results indicate that the cross-plane phonon mean free path of graphite is above 400 nm at room temperature.