Thin and high-quality SSZ-13 membranes were synthesized by the steam-assisted conversion (SAC) method for the first time. More than 95% gel/solution was saved for membrane growth compared with conventional hydrothermal synthesis. Another advantage for the SAC method was that the competitive crystallization between the bulk solution and support was eliminated. The thickness of SSZ-13 membranes was well controlled from 1.2 to 3.0 mu m by the dilution of sucked solution and synthesis time. Three membranes prepared under optimized conditions displayed N-2 permeances of (1.4 +/- 0.12) x 10(-7) mol s(-1) pa- (similar to 418 GPU), N-2/CH4 selectivities of 14.8 +/- 0.45 and a separation index of 0.19 +/- 0.02 mol m(-2) s(-1) at 298 K, and 0.2 MPa pressure drop for an equimolar N-2/CH4 mixture. The membranes synthesis by the SAC method had a good reproducibility. Such separation performances of current SSZ-13 membranes were beyond the upper bound for N-2/CH4 separation among the reported membranes. The influences of temperature, pressure drop, and feed flow rate on separation performance of the membranes were investigated. High-pressure N-2/CH4 separation was also reported using SSZ-13 membranes at the pressure drop up to 2.5 MPa.