Olefin Block Copolymer (OBC)/Multi-walled Carbon Nanotubes and OBC/graphene nanocomposites obtained by solution mixing with good dispersion both display superior reinforcement and toughening in mechanical properties with tensile strength increase from 6.58 MPa to 11.23 MPa and 8.78 MPa relatively with filler content of 1%, and elongation at break can be as high as 2610% and 2780% relatively compared to 1780% of neat OBC. The reinforcement mechanism is studied that the incorporation of nano fillers have significant affect on sliplink contribution at low strains (<200%), and yield-like plateaus in low strain region brought by interfacial interaction between OBC crystals and nano fillers are remarkably higher. The nanocomposites also showed notable improvement in thermal stability with the temeperatures at the peak sample weight loss rates 22 ℃ higher than that of neat OBC. Nucleating ability of MWCNTs and graphene in OBC is studied by DSC nonisothermal study showing that the MWCNTs have stronger inducing ability because of size-dependent soft-epitaxy mechanism. Crystal structure is also studied by wide-angle X-ray diffraction, small-angle X-ray scattering and polarized optical microsopy, giving evidence that the nanocomposites showed increase in lamellar thickness with no change in crystal form.