The study of fracture mechanics applied to rocks has been developed since 1960. The main parameter, fracture toughness, quantifies the rock ability to absorb energy until it ruptures. This knowledge is very important in several areas of rock engineering, such as stability analysis of rocky slopes, rock clearing processes, mining excavations, hydraulic fracturing, earthquake mechanic, among others. The study presents and analyzes the influence of the weathering class in the mode I fracture toughness of gneiss rocks in the city of Rio de Janeiro, using Cracked Chevron Noched Brazilian Disc (CCNDB). Samples of different classes were characterized for their mineralogical, physical (porosity and dry specific weight) and mechanical properties (point load strength, uniaxial compressive strength and tensile strength). The obtained results allowed correlations to be made between toughness and the other analyzed properties. By doing this, it was possible to verify that the weathering class of a rock is directly proportional to the porosity and inversely proportional to the strengths (uniaxial compressive and tensile). Thus, the highly weathered rock presents higher porosity and lower strength when compared to less weathered rocks (sound and moderated). The fracture toughness for moderately weathered rocks is 45,9 percent lower than sound rocks, and 86,7 percent smaller for highly weathered rocks. Therefore, the class of weathering rocks influences all the analyzed properties in a significant way.