The effect of multi-wall carbon nanotubes (MWCNTs) addition in co-polyimide Hollow Fiber Membranes (HFMs) on their mechanical behavior was investigated. BTDA-TDI/MDI (P84) co-polyimide hollow fiber membranes, both single phase and composites, were prepared by wet spinning process via phase inversion technique. Phenol-functionalized MWCNTs were used as filler material into the polymer matrix, while carbon HFMs (CHFMs) were produced by controlled pyrolysis. MWCNTs addition significantly increased tensile and flexural modulus of elasticity of the HFMs. A linear increase of ultimate tensile strength along with the filler concentration reflects the good MWCNTs dispersion in the HFMs. This tensile and flexural strength increase was also followed by an increase in helium and nitrogen permeances as well as by a decrease in tensile ductility. The transition of the polymeric to the carbon phase resulted in a dramatic increase in modulus of elasticity and a linear correlation between ultimate tensile strength and filler concentration was also observed. The mechanical test results and the effect of the filler concentration are discussed by taking into account the gas permeance performance of the investigated membranes. (C) 2017 Elsevier B.V. All rights reserved.