The SiO2/H2O ratio of the synthesis solution has high importance in the synthesis of selective DD3R zeolite membrane. A DD3R membrane (M1) has been synthesized by procedure with a defined low SiO2/H2O ratio used mostly for the synthesis of DD3R powder. Another membrane (M2) has been synthesized by procedure with a defined high SiO2/H2O ratio that the synthesis of high selective DD3R membrane with this method is limited to NGK1-insulators. XRD and SEM confirmed the formation of DD3R layer on the support with both methods. The surface of membranes were treated with polydimethyl siloxane solution. The results of pure gas permeation of CO2, N-2 and CH4 and CO2/CH4 equimolar mixture showed that using the synthesis procedure with the low SiO2/H2O ratio leads to DD3R membrane with less defects, while the obtained membrane with the high SiO2/H2O ratio was so thick with low performance. Also, high separation performance of the membrane M1 (selectivity of 290 and 9 for CO2/CH4 and N-2/CH4) reveals that the procedure with the low SiO2/H2O ratio following by the surface treatment with PDMS solution can be used as a high potential procedure to synthesize a selective DD3R zeolite membrane for separation of CO2 and N-2 from CH4. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.