Nanochannel size control by using layer-by-layer (LBL) deposition of polyelectrolytes is experimentally studied in single hard Polydimethylsiloxane (PDMS) nanochannel. Electroosmotic flow (EOF) velocity is measured by using the current-slope method in the modified nanochannel with various numbers of the deposition layers. The results reveal that the growth of polyelectrolyte layer in nanochannels is quite different from that on flat substrates. In small nanochannels, the increment of the multilayer thickness will be limited unlike that on flat surfaces. As the number of the deposition layers increases, the nanochannel size will decrease initially and reach a saturation value finally where the nanochannel cannot be filled with the polyelectrolytes. It is observed that both the decrement and the saturation value of the nanochannel size strongly depend on the concentration of the salt addition in polyelectrolyte solutions. At low ionic concentration, increasing the salt addition increases the average layer thickness within the nanochannels and decreases the saturation value of nanochannel size. However, the effects of salt addition can demonstrate opposite behaviors at high ionic concentration. The EOF can be alternatively reversed in the modified nanochannels by repeatedly coating oppositely charged polyelectrolytes, and the EOF velocity will gradually decrease with the increasing deposition cycles. (C) 2019 Elsevier Inc. All rights reserved.