SiC/SiC composites have aroused interest for nuclear applications thanks to their high decomposition temperature, strong thermomechanical properties, neutron transparency and low swelling under irradiation. Given these properties, SiC/SiC composites are mainly considered as cladding material either for generation IV systems (such as Gas-cooled Fast Reactors) or current nuclear power plant (Light Water Reactors). Two types of fibers are currently available to manufacture nuclear grade composites Hi-Nicalon S (HNS) and Tyranno SA3 (TSA3). Unfortunately, CVI matrix composites made of each fiber do not exhibit similar mechanical behavior. The HNS-based highlight higher ultimate tensile strains and stresses than the TSA3-based ones. Thus, both fibers have similar properties and identical interphase and matrix, which should have lead to similar behavior. The fiber/matrix coupling must have a strong influence on the final mechanical behavior. Hypotheses were made that those differences came from discrepancies in the fibers surface roughness but its decrease is not the only factor at stake. The understanding of the mechanisms controlling the fiber/matrix were then conducted in order to improve the mechanical properties of SiC/SiC composites made of TSA3 fibers. The fiber/matrix interface was characterized by a combination of mechanical testing and TEM investigations. The differences in the composition of the fiber surface were quantified by XPS and IGC. Results have pointed out that the carbonaceous structure of the fiber extreme surface has to be responsible of those mechanical behaviors.