Experiments　were　performed　in　a　90-L　vessel　with　a　side　vent　to　investigate　the　effects　of　transverse　concentration　gradients　on　the　vented　explosion　of　non-uniform　methane-air　mixture.　Controlled　by　a　mass　flow　meter,　methane　was　injected　uniformly　into　the　vessel　through　18　nozzles　that　are　evenly　distributed　at　the　top　to　configure　a　methane-air　mixture　with　an　average　methane　volume　fraction　of　10.9%.　Various　transverse　con-centration　gradients　were　obtained　by　altering　the　free　diffusion　time　of　the　methane　in　the　vessel.　The　deflagration　flame　was　recorded　through　a　high-speed　camera　at　a　frequency　of　1000　Hz,　and　the　internal　overpressure　at　different　locations　was　captured　through　three　piezoelectric　sensors.　Experimental　results　reveal　that　larger　concentration　gradient　resulted　in　lower　flame　propagation　rate,　longer　time　for　the　burst　of　the　vent　cover,　and　stronger　pressure　oscillations.　Concentration　gradient　affects　the　typical　flame　structure,　especially　the　formation　and　development　of　the　tulip　flame.　Under　the　present　experimental　conditions,　Helmholtz　oscillations　occurred　in　all　tests.　Concentration　gradient　has　a　negative　correlation　with　the　frequency　of　Helmholtz　oscillations　but　a　positive　correlation　with　the　duration　of　oscillations.　In　conclusion,　the　variation　of　concentration　gradient　obviously　affects　flame　propagation　and　pressure　evolution　during　vented　explosions,　which　deserves　more　attention　in　the　design　of　explosion　protection.