This paper reports on the relative role of the functional group content and the molecular weight of a glycidyl methacrylate (GMA) functionalised PP (PP-g-GMA) as reactive compatibiliser in the compatibilisation of polypropylene (PP) and poly(butylene terephthalate) (PBT) immiscible blends in a co-rotating twin screw extruder. Two series of PP-g-GMA were compared in terms of their compatibilising effects on the morphology and mechanical properties (elongation at break and impact strength) of the blend. The first series of PP-g-GMA had the same molecular weight but different GMA contents while the second series of PP-g-GMA had the same GMA content but different molecular weights. The compatibilising effects of the first series of PP-g-GMA were dictated primarily by the amount of GMA in the blend, i.e., two PP-g-GMA having different GMA contents had the same compatibilising efficiency when the amount of GMA in the blend was the same. For the second series of PP-g-GMA, the higher the molecular weight, the higher the compatibilising efficiency.