The layer thickness of a zeolitic imidazolate framework-8 (ZIF-8) membrane was successfully reduced in order to obtain high permeance of propylene. The ZIF-8 membranes for propylene/propane separation were prepared using a counter-diffusion method by changing the solution concentration and the molar ratio of 2-methylimidazole to zinc ion (Hmim/Zn²⁺). ZIF-8 layers with different thicknesses were formed in the outermost regions of porous α-alumina capillary substrates, and the minimum thickness of the ZIF-8 layer was 5 µm. Single-component gas permeation properties were measured using propylene and propane at a temperature of 298 K, and the permeability, diffusivity, and solubility were analyzed. The maximum propylene/propane permselectivity of 135, diffusivity selectivity of 125, and solubility selectivity of 1.1 were obtained at a Hmim/Zn²⁺ ratio of 1 with a propylene permeance of 1.1×10⁻⁹ mol·m⁻²·s⁻¹·Pa⁻¹. By contrast, a propylene/propane permselectivity of 113 with a propylene permeance of 3.3×10⁻⁹ mol·m⁻²·s⁻¹·Pa⁻¹ was obtained at a Hmim/Zn²⁺ ratio of 0.33. The thickness of the ZIF-8 layer, the permselectivity of propylene/propane, and the permeance of propylene were controlled by regulating the concentration and the Hmim/Zn²⁺ ratio for the synthesis of the ZIF-8 membrane by the counter-diffusion method.