The concept of fiber reinforced concrete (FRC) was invented to reinforce concrete with a wide range of fibers such as steel fibers, glass fibers, synthetic fibers, polyethylene fibers or carbon fibers. The addition of superplasticizers (SP) has enabled to gradually reduce W/B ratio and improves binder dispersion, which in turn improves mechanical properties of the cementitious matrix. In this study, two different types of fibers were mixed. It was studied the workability of cementitious mortars and the impact of fiber length and dosage. Natural fibers such as hemp, miscanthus and flax fibers were mixed with FIBRAFLEX fibers. FIBRAFLEX fibers are amorphous metallic fibers resistant to corrosion and chemical attacks. Two different sizes of fibers (FF13E7, FF15E0) were mixed with natural fibers in six different formulations. The composition of the granular skeleton and fibers have an impact on the workability of different formulations, and this aspect was studied using the slump cone. For the analysis of the cementitious matrix and the fiber/matrix interface, tomography was used to analyze the microstructure, observe the natural fiber/FIBRAFLEX fibers/cementitious matrix interaction and analyze material heterogeneity and fiber tortuosity within the cementitious matrix. Mechanical strength tests at 1, 7 and 28 days using 3-point bending and compressive strength tests were carried out, enabling us to gain a better understanding of the material characteristics obtained via tomography tests. This study investigates how fiber affects mechanical properties.