We investigated the phonon-limited electron mobility in rectangular silicon nanowire metal-oxide-semiconductor field-effect transistors (Si NW MOSFETs) using the Kubo-Greenwood formula. Although cross-sectional electrostatics has been actively investigated, the cross-sectional distribution of the spatially resolved mobility has not been studied yet. In this paper, we discuss the corner effects based on spatially resolved mobility analysis. Taking into account the subband composition for local electrons, we could study the cross-sectional spatially resolved carrier transport. A strongly confined Si NW MOSFET showed that mobility hardly fluctuated in the cross-sectional distribution with strong volume inversion. On the other hand, a less-confined Si NW MOSFET showed that the corner mobility was lower than the side mobility; therefore, our result could imply that the corner effects were not positive for improvement in mobility.