Two magnesium hydroxide fillers of different morphology, but almost identical surface area, were incorporated into polypropylene to assess their nucleation properties. The platelet filler was found to reside parallel to the flow direction, whereas the pseudospherical filler resided isotropically within the matrix. The plate plane (001) of magnesium hydroxide was found to be responsible for causing nucleation, the polypropylene b axis growing parallel to the melt flow direction. Vinyl silane modification reduced this effect. Filler loading level was found to be influential in governing crystallization initiation, growth, and final crystallinity levels. Law filler loadings produced fast crystallization rates but low crystallinity contents, whereas high filler contents produced the opposite effects. Mechanical properties were found to be influenced by matrix morphology; however, filler content became the major factor at loading levels of 40% and above.