In the presented work the effect of the structure of multiwalled carbon nanotubes (MWCNTs) as well as the processing conditions during compression and injection moulding on the morphology, percolation and mechanical behaviour of isotactic polypropylene-based/MWCNT composites was investigated. Melt mixed composites are composed of different MWCNT materials (0-5 wt.%) and a maleic anhydride-modified isotactic polypropylene (iPP-MA) matrix. MWCNT modification included ball milling and reactive ball milling of the as-synthesised MWCNT material. Structure-property-relations were studied by means of spectroscopic and microscopic methods, X-ray scattering, thermal analysis, tensile testing, and modelling of the MWCNT reinforcement effect. Percolation thresholds and reinforcement efficiency of the MWCNT materials originated from their different length and primary agglomerate structure. The crystalline morphology of the iPP-MA resulting from the incorporated MWCNTs and the applied processing conditions dominated the isotropic and anisotropic mechanical behaviour of the composites. Furthermore, the state of MWCNT macroscale dispersion stemmed from the size of their primary agglomerates and determined the ductility of the composites, whereas the stiffness was influenced by the MWCNT structure. In summary, it was shown that the MWCNT structure as well as the conditions during composite processing significantly determined the structure-property-relations in the iPP-MA/MWCNT composites.