The research work comprehensively evaluates and compares the strengthening efficiency of multiwalled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs), two of the most promising nano-size reinforcements, which could be employed for fabricating high strength-low weight nanocomposites. High energy ball milled Al powder and different loadings of CNT and GNP (0.5 and 1.0 wt%) were mixed and subsequently consolidated via spark plasma sintering to synthesize Al-CNT and Al-GNP nanocomposites. A clean Al-CNT/GNP interface devoid of traces of any micro-pores and reaction products was achieved. Al-1.0 wt% CNT/GNP showed lower crystallite size and relative density as compared to Al-0.5 wt% CNT/GNP counterpart. Al-1.0 wt% GNP exhibited the lowest relative density (96%) owing to very high volume and surface area of GNP which resulted in rapid agglomeration. Al-CNT nanocomposites exhibited better compression properties as compared to Al-GNP nanocomposites. During compression, for 0.5 wt% reinforcement, the reinforcing efficiency of CNT was 12.4, whereas, that of GNP was 53. Tensile test revealed that as compared to milled Al powder compact, Al-0.5 wt% CNT and Al-0.5 wt% GNP exhibited 70% and 85% higher yield strength, respectively. The contributions from different strengthening mechanisms were calculated to predict the yield strength of the sintered nanocomposites. It was found that, for 0.5 wt% reinforced CNT and GNP, the experimentally obtained yield strength of respective nanocomposites were about 91% and 97% closer to the predicted values confirming the effective translation of different strengthening mechanisms towards the strength of the nanocomposites. Further increase in the CNT/GNP content to 1.0 wt% led to deterioration in the compression and tensile properties due to extensive aggregation of respective reinforcements in the Al matrix and the extent of deterioration was substantial for Al-1.0GNP. (C) 2019 Elsevier B.V. All rights reserved.