The reinforcing efficiency of polymer whiskers of poly(4-hydroxybenzoate) (PHB) in polyamide-6 (PA-6) and polyamide-11 (PA-11) composites was investigated by tensile testing, scanning electron microscopy (SEM), and density measurements. The composites were prepared by mixing the whiskers into the melts of the polyamides. Ductile and stiff PA-6-composites with different crystallinities of the PA-6-matrix were investigated. In the ductile PA-6 (low crystallinity) composites, the elastic modulus E and the yield stress sigma(f) increase more than twice at a whisker content of 2 vol %. The increase in E and sigma(f) in stiff PA-6 (high crystallinity) composites is not as pronounced as in the ductile PA-6 composites, but is still remarkably higher than in the PA-11 composites, which is about 1.2 times at a whisker content of 2 vol %. At higher whisker content, the PA-6 composites show the opposite of the PA-11-composite-no further increase in E and sigma(f), which may be due to an agglomoration of whiskers in the high viscous PA-6 melt. The different tensile properties of the composites can be explained by SEM analysis of the fracture surfaces, which shows that the adhesion of PA-6 to the PHB whiskers is better than of PA-11. This is due to a higher number of hydrogen bonds between the PA-6 and the whisker surface. Density measurements show that the crystallinity of the polyamides is not affected by the PHB whiskers.