In this study, biomass cokes from sunflower seed hull (SFSH) and wood pellets (WP) were added to a MgO-C batch (3 mass% C) to replace 1.1 mass% of graphite. After hardening and carbonizing the samples, the influence of the biomass cokes on the microstructure and thermal shock resistance was investigated. The replacement of flaky graphite by carbonized WP and SFSH reduced the bulk density and increased the apparent porosity after pressing and carbonization, but the degree was only marginal. This was confirmed by SEM investigations, where the biomass-coke containing samples exhibited a microstructure with a higher amount of pores between the fine MgO grains. The thermal shock resistance of the porous wood pellet coke containing MgO-C is at the same level as the reference sample but not superior to it.