The paper presents an analysis for the relative slip amount related to the fluid-wall interfacial shear strength in a nano channel formed by two parallel smooth solid planes sliding against one another. A closed-form equation formulation for the relative slip amount was obtained. It was shown that the relative slip amount is constant in the inlet zone of the channel, where the interfacial slippage occurs; This substantiates the assumption made in the analysis of this kind of channel by Zhang (2015). It was also shown that with the increases of the channel height or the fluid-wall interfacial shear strength, the interfacial slippage is reduced; The strong fluid-stationary wall interaction in the outlet zone of the channel results in more severe interfacial slippage than the medium-level fluid-stationary wall interaction in the outlet zone.