Thin and oriented zeolite films are important for advanced separations, catalysis, and sensing. Strategies for tailoring zeolites for applications include controlling their crystal size, shape, and orientation. Here, three siliceous DDR zeolite ultrathin films with different orientations achieved by homoepitaxial growth from 60 nm-sized seed particles are reported. The 0.5 µm thick membrane shows a separation selectivity of 400 for CO₂–CH₄ mixtures and CO₂ permeance of 25 × 10^-7 mol m^-2 s^-1 Pa^-1 at 20 °C and 1 bar, leading to a record-high performance among all reported DDR membranes. Furthermore, the seed nanoparticles are grown into mono-dispersed DDR sub-micron crystals with trigonal and tabular habits. These crystals are assembled in monolayers for the growth of ultrathin and uniformly (h0h)-oriented and c-oriented films with maximum surface pore diameter of 0.365 and 0.263 nm, respectively, by using the 1-adamantanamine template in fluoride medium. The novel strategy not only provides high-performance membrane candidates for industrial CO₂ separation, but also inspires interfacial engineering, pore size, and orientation controlling for other microporous crystals, and their membranes.