In this work, the structure relationships of the blend morphology and the fracture mechanical behavior of immiscible polyamide 6 blends are investigated at low testing speeds. Typical investigations in this field are based on impact tests, which are several orders of magnitude faster than fracture mechanical tests performed in this investigation. It could be shown that the known criteria for impact tests of the optimum particle size and the critical interparticle spacing are not applicable for low testing speeds. In any case, a good phase connection between the blend partners is a basic requirement. By means of the method of essential work of fracture, it is shown that the increase in toughness depends only on the particle size. Therefore, the smaller the soft phase particles within the matrix for a given material combination, the higher the toughness increase.