The performance of mixed-matrix membranes (MMMs) based on Matrimid and benzimidazole-linked polymers (BILPs) have been investigated for the separation CO/N and the dependency on the filler porosity. BILPs with two different porosities (BILP-101 and RT-BILP-101) were synthesized through controlling the initial polymerization rate and further characterized by several techniques (DRIFTs, C CP/MAS NMR, SEM, TEM, N and CO adsorption). To investigate the influence of porosity, the two types of fillers were incorporated into Matrimid to prepare MMMs at varied loadings (8, 16 and 24 wt%). SEM confirmed that both BILP-101 and RT-BILP-101 are well dispered, indicating their good compatibility with the polymeric matrix. The partial pore blockage in the membrane was verified by CO adsorption isotherms on the prepared membranes. In the separation of CO from a 15:85 CO:N mixture at 308 K, the incorporation of both BILPs fillers resulted in an enhancement in gas permeability together with constant selectivity owing to the fast transport pathways introduced by the porous network. It was noteworthy that the initial porosity of the filler had a large impact in separation permeability. The best improvement was achieved by 24 wt% RT-BILP-101 MMMs, for which the CO permeability increases up to 2.8-fold (from 9.6 to 27 Barrer) compared to the bare Matrimid.