The normalization method is applied to different magnesium hydroxide filled polypropylenes. As the load separation principle is the basis of the method, its validity is checked using the load separation criterion developed by Sharobeam and Landes. Load separability is checked for all the materials when the condition of stationary crack length is fulfilled. During the determination of the deformation function using the normalization method, the large decrease of the load value of highly filled materials make it impossible to describe the load normalization variation with plastic displacement by a power law equation. Nevertheless, for the lower-filled materials, i.e. up to 40 wt% with copolymer PP and 20 wt% with homopolymer PP, the J-R curve can be determined and high concordance is found with the J-R curve obtained by multiple specimen method. The applicability of the normalization method is discussed in terms of the geometry of the plastic deformation zone. It is found that the materials that are not suitable for normalization method application are characterized by a very small plastic zone, due to the restriction of plastic flow caused by mineral filler.