For the evaluation of CH4/N-2 separation, mixed-matrix membranes (MMMs) were fabricated by adding [Ni-3(HCOO)(6)] as a filler into the poly(styrene-b-butadiene-b-styrene) (SBS) polymer. The structures and morphologies of [Ni-3(HCOO)(6)] and [Ni-3(HCOO)(6)]/SBS MMMs were characterized by different techniques, proving the crystalline structure of [Ni-3(HCOO)(6)] in SBS polymer matrices. The structural integrity of the MMMs and good compatibility between [Ni-3(HCOO)(6)] and SBS polymer were examined by scanning electron microscopy. Gas permeation measurements revealed that membranes containing various [Ni-3(HCOO)(6)] loadings exhibited different separation performances. The operational temperature exerted a significantly positive effect on the separation of CH4 and N-2.