In this work, we try to understand the viscous effects of the outer potential flow on Prandtl's boundary layer. For a body moving with a constant velocity in an otherwise quiescent liquid, the non-zero viscous dissipation of the outer potential flow gives rise to an additional drag, increasing the drag calculated from the boundary layer flow alone. The discrepancy of the shear stress at the outer edge of the boundary layer induces a viscous pressure correction of the irrotational pressure. The pressure correction varies inside the boundary layer and has viscous effects on the body, which are not captured in Prandtl's boundary layer theory. These viscous effects of the outer flow on Prandtl's boundary layer is small when the Reynolds number is high.