Carbon nanotubes (CNTs) were dispersed into Al matrix by multi-pass friction stir processing (FSP) to fabricate 4.5vol.% CNT/2009Al composites. The maximum strength of the composites was obtained with three-pass FSP and was attributed to the combined effect of CNT cluster reduction, grain refinement, and CNT shortening. A CNT shortening model is proposed to describe the CNT length change with FSP passes. The model indicates that the reciprocal of the CNT length has a linear relationship with the duration of the mechanical effect, which was verified by our and other investigators’ experimental results on various mechanical processes. Based on the concept of the load transfer efficiency of CNTs in the Al matrix, a universal strength model considering the microstructural parameters – the aspect ratio of CNTs, grain size, and concentration of CNT clusters and pores – is proposed to predict the strengthening of the CNT/2009Al composites with and without CNT clusters. The predictions are in good agreement with the experimental results.