Sulfonated fluorenyl-containing polyaryletherketones (SFPAEKs) were synthesized through a postsulfonation approach under a mild reaction condition. The composite proton exchange membranes based on SFPAEKs and various amounts of the modified nanocrystalline cellulose (NCC) were prepared by solution casting method. The existence of the multiple hydroxyl and sulfonic acid groups on the chemically modified nanocrystalline cellulose was supposed to benefit the formation of hydrogen-bond network and proton-conducting channels, which would improve the proton conducting ability of the composite membranes. Furthermore, the properties, such as mechanical properties, thermal stability, water uptake, swelling ratio and so on, were thoroughly investigated. In comparison to the pristine SFPAEK, the composite membranes containing a "performance enhanced" NCC component presented the higher proton conductivity and better mechanical properties. It was found that the proton conductivity of the composite membrane with 4 wt% of NCC could reach 0.234 S cm(-1) at 100 degrees C, and this value was higher than that of most of the reported membranes. The results showed that the modified nanocrystalline cellulose reinforced SFPAEK composite membrane would be promising for the application as middle-temperature proton exchange membranes in fuel cells. (C) 2016 Published by Elsevier B.V.