We　theoretically　calculate　the　electron　mobility　in　silicon　nanowire　metal-oxide-semiconductor　field-effect　transistor.　The　calculation　of　electron　mobility　is　based　on　the　Kubo-Greenwood　formulation　and　the　self-consistent　solutions　of　Schrodinger　and　Poisson　equations.　Phonon　scattering　(PHS),　Coulomb　scattering,　and　surface　roughness　scattering　(SRS)　mechanisms　are　considered　in　this　study.　The　nonlinear　SRS　model　is　employed　because　it　can　reproduce　the　experimental　mobility　with　realistic　surface　roughness　parameters.　The　simulation　demonstrates　that　PHS　and　SRS　are　both　vital　scattering　mechanisms.　The　rate　of　SRS　significantly　increases　with　decreasing　device　size,　and　SRS　is　the　dominant　scattering　mechanism　in　ultra-narrow　silicon　nanowires.