Carbon nanotube reinforced aluminum matrix (CNT/Al) composites with various microstructures were fabricated by powder metallurgy via introducing nano-sized and micron-sized Ti powders into the CNT-Al system. The size-dependent effects of Ti powders on the microstructure evolution and mechanical properties were further investigated. The results reveal that the best enhancement (∼51% of UTS) was obtained by adding 80 nm sized Ti powders into the CNT/Al composite compared with the composite without Ti addition, which mainly derives from the synergistic strengthening of the homogenously dispersed CNTs, a large quantity of in-situ formed nano-sized TiAl3 particles and fine grains. Meanwhile, it is also noted that the addition of micron-sized Ti powders can help the CNT dispersion in powder fabrication stage, and subsequently in-situ formed core-shell structured particles with different interface reaction degree in the Al matrix. Among them, 1 μm was determined to be the optimal size of Ti for the maximum formation of core-shell structured particles, which best exploits the load transfer ability of TiAl3 shell layers at their interfaces, and finally attributes to significant strength enhancement (∼35% of UTS). This research provides a valuable reference concerning the size effects of addition metal in future fabrication of CNTs reinforced metal matrix composites with alternative microstructure. Display omitted •(CNT-Ti)/Al composites with alternative microstructure were fabricated.•CNT/Al-80 nm Ti composites were strengthened by fine grains, CNTs and in-situ formed TiAl3 particles.•Best enhancement effect and balanced ductility were achieved in the CNT/Al-80 nm Ti composites.•Decrease the size of Ti powders can significantly benefit their enhance effect in CNT/Al composites.