This study focuses on the effects of fuel structure and in particular its spatial heterogeneity in the context of fuel-break design. The coupled atmospherewildfire behaviour model HIGRAD/FIRETEC is used to simulate wind fields and fire propagation in a complex landscape including forest-to-break and breakto- forest transitions. Two different Mediterranean ecosystems are used here: a Pinus halepensis (light canopy) and a Pinus pinaster (dense canopy). In both ecosystems, two forest zones are separated by a 200 m break. The study is separated into two parts. First, the break-induced winds are simulated with FIRETEC. The impact of the break structure (cover fraction, clump size) on the mean wind and turbulence statistics are shown. A significant increase of wind velocity and turbulence amount is observed when the cover fraction is reduced within the break. In addition, at low cover fraction, the introduction of tree clumps also induces wind acceleration. In the second part of the study, a fire line is ignited in the area upwind of the break and the fire propagation is computed using the precomputed wind fields of the first part of the study. The fire propagates in the upwind forest area before crossing the break and propagating in the downwind forest area. A decrease of fire intensity occurs after several meters of propagation on the fuel-break. This intensity decrease is significant when the cover fraction is lower or equal to 25%, but negligible at 50%. In addition, in the Pinus pinaster canopy, the fuel structure and especially clump size affect the fire damage