Since　the　control　system　in　a　permanent　magnet　synchronous　motor　(PMSM)　has　uncertainties　and　disturbances,　the　switching　gain　is　a　key　factor　in　effecting　chattering　in　sliding　mode　control.　An　adaptive　switching　gain　is　proposed　for　complementary　sliding　mode　control　(CSMC)　to　regulate　the　PMSM　velocity.　The　proposed　switching　gain　is　designed　according　to　system　uncertainties.　When　the　value　of　the　sliding　modulus　is　outside　the　boundary　layer,　it　is　positive,　resulting　in　an　increase　in　the　switching　gain.　Conversely,　when　it　is　negative,　the　gain　is　decreasing.　Further,　the　proposed　law　considers　an　arbitrary　small　vicinity　of　the　sliding　manifold,　in　which　the　derivatives　of　the　adaptive　switching　gains　are　inversely　proportional　to　the　sliding　modulus　to　speed　up　the　gain　adjustment　process.　The　proposed　gain　for　complementary　sliding　mode　control　is　applied　to　regulate　the　PMSM　velocity;　comparisons　of　experimental　results　show　that　the　proposed　gain　has　a　faster　gain　adjustment　process　than　CSMC　and　alleviates　chattering,　resulting　in　a　greater　velocity　accuracy.