This　work　proposes　a　design　principle　of　the　optimal　thickness　for　cathode　gas　diffusion　layers　(GDLs)　of　proton　exchange　membrane　fuel　cells　(PEMFCs)　based　on　a　balance　of　cell　performances　under　both　steady-state　and　load-varying　conditions.　Using　a　three-dimensional　and　two-phase　model,　the　effects　of　cathode　GDL　thickness　(50,　100,　200,　and　400　mu　m)　on　steady-state　performances　and　transient　response　characteristics　are　studied.　The　results　show　that　under　steady-state　conditions,　an　extremely　thin　cathode　GDL　would　lead　to　non-uniform　oxygen　and　liquid　water　distributions,　whereas　an　especially　thick　cathode　GDL　would　increase　oxygen　transport　resistance.　Thus,　there　is　an　optimal　cathode　GDL　thickness,　yielding　the　best　steady-state　performance.　A　current　overshoot　phenomenon,　arising　from　high　local　oxygen　concentrations　at　cathode　catalyst　layers　(CLs),　is　observed　when　the　load　voltage　decreases　abruptly.　Thinner　cathode　GDLs　would　weaken　the　overshoot　and　shorten　the　recovery　time　of　current　density.　Conversely,　low　local　oxygen　concentrations　at　cathode　CLs　caused　by　high　liquid　water　saturation　levels　lead　to　a　current　undershoot　phenomenon　when　the　load　voltage　increases　abruptly.　Thinner　cathode　GDLs　would　strengthen　the　undershoot　but　shorten　the　recovery　time　of　current　density.　The　optimal　choice　of　cathode　GDL　thickness　should　weigh　up　the　steady-state　and　the　transient　performance.　Based　on　this　principle,　the　optimal　cathode　GDL　thickness　is　selected　as　100　mu　m.