Experiments　were　conducted　in　a　1　m(3)　vessel　with　a　top　vent　to　investigate　the　effect　of　methane　concentration　and　ignition　position　on　pressure　buildup　and　flame　behavior.　Three　pressure　peaks　(p(1),　p(2),　and　P-ext)　and　two　types　of　pressure　oscillations　(Helmholtz　and　acoustic　oscillations)　were　observed.　The　rupture　of　vent　cover　results　in　p(1)　that　is　insensitive　to　methane　concentration　and　ignition　position.　Owing　to　the　interaction　between　acoustic　wave　and　the　flame,　p(2)　forms　in　the　central　and　top　ignition　explosions　when　the　methane-air　mixture　is　near-stoichiometric.　When　the　methane-air　mixture　is　centrally　ignited,　p(2)　first　increases　and　then　decreases　with　an　increase　in　the　methane　concentration.　The　external　explosion-induced　P-ext　is　observed　only　in　the　bottom　ignition　explosions　with　an　amplitude　of　several　kilopascals.　Under　the　current　experimental　conditions,　flame-acoustic　interaction　leads　to　the　most　serious　explosions　in　central　ignition　tests.　Methane　concentration　and　ignition　position　have　little　effect　on　the　frequency　of　Helmholtz　and　acoustic　oscillations;　however,　the　Helmholtz　oscillation　lasts　longer　and　first　decreases　and　then　increases　as　the　methane　concentration　increases　for　top　ignition　cases.　The　ignition　position　significantly　affects　the　Taylor　instability　of　the　flame　front　resulting　from　the　Helmholtz　oscillation.