The effect of biomass (Eucalyptus globulus, pine cones, and Brassica rapa) composition on the gamma-irradiation behavior of some polypropylene (PP) biocomposites has been studied. The variations in mechanical properties, contact angles, crystallinity index, hydroxyl and carbonyl indices, and induction period of the oxidative process indicated a strong influence of biomass composition on the behavior of biocomposites under gamma-irradiation. The high gamma-irradiation dose influences the properties of PP biocomposites. The contact angle values of irradiated samples decreased with respect to unirradiated PP/biomass composites, indicating an improved surface hydrophilicity. The chemiluminescence study revealed the capability of biomass to delay long-term oxidative degradation. The durability of these products is enhanced because of the protective role of lignin, which acts as an antioxidant due to its phenolpropanoic structure. The lignin activity depends on its content in biomass incorporated in polypropylene. Therefore, according to the increase in antioxidant quality, the studied biomasses were placed in the sequence Eucalyptus globulus similar to pine cones > Brassica rapa, which corresponds with decreased lignin content in biomass samples.