Hybrid composites of rice straw (Rs)/seaweed (Sw) and polypropylene (PP) were prepared at a fixed filler ratio of 30:70 and variable ratio of the two reinforcements, viz. 30:0, 25:5, 20:10, 10:20, 0:30 by weight. Mechanical properties of the composites such as tensile strength (TS), bending strength (BS), impact strength (IS) and elongation at break (Eb%) were investigated and the composite formulation of 20:10:70 (Rs:Sw:PP) was found to be optimum that showed TS = 2.8 MPa, BS = 68 N/mm(2), IS = 2.5 kJ/mm(2) and Eb = 50%. For better compatibility, Rs and Sw were subjected to surface treatment using various intensities of gamma-radiation to prepare improved hybrid composites gamma-irradiated filler hybrid composites significantly enhanced mechanical properties and the composite in which fillers were irradiated at 100 krad achieved maximum enhancement with TS = 35 MPa, BS = 75 N/mm(2), IS = 2.7 kJ/mm(2) and Eb = 68%. Water absorption capacity of the different composites was also studied and irradiated filler composites showed less water uptake. (C) Koninklijke Brill NV, Leiden, 2011