In this work, we report the effect of seed particle size towards the formation of NaX zeolite membranes through single and multi-stage hydrothermal growth methods. A seed layer was firstly deposited onto the α-alumina support via vacuum-assisted seeding technique prior to hydrothermal growth of the membrane layer. The morphology and crystallinity of the membranes were characterized by using scanning electron microscope (SEM) and X-ray diffraction (XRD), respectively. From the results obtained, by using smaller seed particles of 750 nm, the growth of the membrane layer was enhanced due to minimum gaps found in between the seed particles. The N2 gas permeation results showed that the multistage synthesis method has successfully reduced the defects of the membranes. However, it was observed that the presence of zeolite NaP impurities in the membrane layer became more significant along with the increase of the synthesis stage. © 2021 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of The Korean Ceramic Society and The Ceramic Society of Japan.