Currently, the structural integrity of the nuclear graphite reactor components is evaluated using irradiated material properties data obtained from test reactors. These data are applied to numerical or mathematical models to assess the integrity of the graphite components. On the other hand, there is a need to measure internal strains and stresses in reactor core graphite and to explore the potential for in-situ measurement. The deep hole drilling (DHD) technique is a semidestructive method for measurement of the through thickness residual strains or stresses. Previously the technique has been applied successfully to metallic and composite materials. In this paper, the method for internal strain and stress measurement in polygranular graphite is examined particularly when a significant volume fraction of porosity is present. Finite element simulation shows that the method can be used to measure internal stresses generated by a thermal gradient. On the basis of this a series of experiments have been conducted using Pile Grade ‘A’ (PGA) and PG25 filter graphite; with the latter being a surrogate for service exposed material. Tests were conducted to illustrate that the bulk mechanical response was linear elastic for both graphites and elastic moduli were measured. The DHD method was then used to determine internal strain and stress profiles through the materials. The results were compared with the strain data obtained from strain gauges bonded to the samples. Overall, there was an excellent agreement between the measured through thickness internal strains and stresses and the surface strains and stresses determined from the applied loading.