Annual growth agrofibers like flax and jute can compete with glass fibers, considering their intrinsic mechanical properties. This paper discusses reinforcement of polypropylene (PP) homopolymer with flax bast fibers. Maleic Anhydride modified PP (MAPP) grades are screened on coupling efficiency. Three mechanisms appear to be of great importance in the effectiveness of interphase modification: (i) the maleic anhydride (MA) content grafted per polymeric PP-chain; (ii) the molecular weight (M-W) of MAPP; and (iii) the miscibility of MAPP/PP. The coupling efficiency of different MAPP grades, defined as the maximum increase in composite strength, has been investigated. It was found that for the homopolymer based MAPP varieties the M-W of the compatibiliser is a more important parameter than the MA content. Low M-W random copolymer-based MAPP varieties were found to be more effective in increasing flexural strength than high M-W block copolymer-based MAPP varieties. Furthermore, a maximum in MAPP efficiency is sometimes seen; adding more coupling agent to the system does not additionally increase the composite mechanical properties. Impact was investigated by measuring the Charpy impact strength. Block copolymer and random copolymer-based MAPP varieties were found to be generally more effective in improving the Charpy impact strength than the homopolymer-based varieties.