The mass flow rate through a micro/nano channel is calculated by a multiscale analysis when the thickness of the adsorbed layer on the channel wall is comparable to the channel height and the interfacial slippage on the adsorbed layer-wall surface interface occurs or not. The calculation is compared with that from conventional continuum flow theory. It is found that when the ratio landabf of the thickness of the adsorbed layer to the film thickness of the intermediate continuum fluid is no more than 0.02, normally the effect of the adsorbed layer is negligible and a (no multiscale) continuum flow theory can calculate the flow rate through the channel in spite of the fluid-wall interaction, whenever the interfacial slippage occurs or not. Both the increase of landabf and the increase of the interfacial slippage enlarges the proportion of the adsorbed layer flow rate to the total flow rate through the channel. For landabf larger than the critical value which is dependent on both the fluid-wall interaction and the interfacial slippage, a multiscale analysis is normally required for calculating the flow rate through the channel by incorporating the adsorbed layer effect. However, for a great interfacial slippage such as for the dimensionless power loss A on the channel for driving the interfacial slippage over 100, a continuum flow theory may be sufficient in the entire range of landabf for calculating the flow rate through the channel by considering the interfacial slippage and ignoring the existence of the adsorbed layer, when the fluid-wall interaction is medium-level or strong.