Membrane-based water reuse and desalination technologies have been increasingly popular as an alternative water resource for mitigating global water scarcity. However, intrinsic limitations of conventional membranes restrict a further advance in membrane-based technologies. Recently, laminar-structure graphene oxide (GO) membranes fabricated by staking GO nanoplatelets have been emerging as a new water purification membrane with overcoming the conventional membrane limitations because of ultra-fast water permeation and precise molecular sieving capability based on ultra-low friction and well-defined nanochannels; to apply laminar GO membranes for ion removal such as desalination, tuning size of GO nanochannels within a sub-nanometer range is required. Herein, we adjusted reduction degree of GO laminates via varying exposure time to hydroiodic acid steam to control the size of GO nanochannels in a sub-nanometer range. Control of the hydroiodic acid vapor exposure time is a facile way to tune the nanochannel size in a sub-nanometer, and consequentially monovalent ion rejection rate can be handled.