Brewers' spent grain (BSG) and ground tire rubber (GTR) were applied as low-cost hybrid reinforcement natural rubber (NR). The impact of BSG/GTR ratio (in range: 100/0, 75/25, 50/50, 25/75 and 0/100 phr) on processing and performance properties of highly filled natural rubber composites was evaluated by oscillating disc rheometer, Fourier-transform infrared spectroscopy, thermogravimetric analysis, scanning electron microscopy, swelling behavior, tensile tests and impedance tube measurements. It was found that increasing content of GTR in NR/BSG/GTR composites accelerate cross-linking reactions during their preparation, which resulted in decrease of scorch time and optimal cure time. Simultaneously, higher content of GTR filler in NR/BSG/GTR composites significantly improved their physico-mechanical, thermal, morphological and acoustical properties. This indicates better compatibility between natural rubber matrix and GTR than with BSG, which is related to correlation between two factors. First factor is obvious differences in particles size and polarity of GTR and BSG, which affected physical interactions into phase boundary between NR matrix and BSG/GTR hybrid reinforcement. Second factor is possible migration of unreacted curing additives and carbon black particles from GTR filler to NR matrix, which played a significant role on processing and final properties of NR/BSG/GTR composites