When fluids are confined in a structure at nanoscale, such as in a na- nochannel. the electric double layers (EDLs) from opposite walls may have strong interactions. This phenomena results in EDL overlap, which leads to significant changes in the electric field and the fluid properties. Electroosmotic flow (EOF) describes the motion of electrolyte due to the existence of an EDL. Ions in electrolyte have finite sizes which become comparable to EDL thickness in nanochannels and thus cannot be neglected during the formulation. Ion size have been shown to influence the extent of EDL overlapping in channel [S. Das and S. Chakraborty, Physical Review E, vol. 84, p. 012501, 2011]. The electrokinetic effects for a system that are not less than lOnm are examined, where van der Waals forces cease to be significant compared to the electrostatic forces. Thus, it is appropriate to apply continuum equations in modeling. For systems below lOnm the continuum approach is not applicable but the understanding based on continuum modeling are still beneficial as a quantitative reference. In this paper, the electroosmotic flow in nanochannels is investigated by using the asymmetric ion size model proposed by [ Y. Han, S. Huang, and T. Yan, Journal of Physics: Condensed Matter, vol. 26, p. 284103, 2014]. The effect of size of ions on the flow behavior in nanochannels is examined. The average electroosmotic mobility is obtained for both the cases of symmetric and asymmetric ion sizes for different nanochannel widths