The aim of this study was to investigate the fatigue crack initiation in silica-filled solution styrene butadiene rubber. Interrupted fatigue tests were performed on adapted hourglass shaped AE2 samples, while the material microstructure was examined for different fatigue steps using a scanning electron microscope (SEM) and a micro-computed x-ray tomograph (μCT). The main findings indicate that fatiguing induced cracks by debonding between silica agglomerates and the rubber matrix as well as by internal fracture of silica agglomerates. Micro cracks were quantified by image analysis as a function of the number of fatigue cycles. It was demonstrated that the cracks grew with an increasing number of fatigue cycles. It was thus concluded that initiation of these cracks as well as their growth leads to a premature end of life of the material. These findings may help the design and development of rubber compounds with improved fatigue resistance.