ABSTRACT: The present paper aims to perform a complex study of the influence of test specimen thickness on the Fatigue Crack Growth (FCG) of natural and synthetic rubber to enhance the circumstantiality of the measuring methodology. In this work, the mini-Pure-Shear (mPS) test specimens (geometry ratio 1/10 “length/width”) at two different thickness of 0.5 and 1.5 mm were studied using the Tear and Fatigue Analyzer. Two different rubber compounds based on NR and SBR, filled with 50 phr of carbon black and common curatives were investigated. Three double notched test specimens of each material and thickness were simultaneously tested under the Gauss pulse loading condition at varied strains. As a result, the higher slopes, represented by the material parameters, m, were determined for the thinner samples independent on material type. Also, significantly lower deviations of FCG values for thinner samples, independent of material was observed. Thus, it was concluded that the lower thickness of the sample represents a more critical loading case, whereby the experimental data can be determined more precisely.