In　this　study,　the　effects　of　vent　size　on　the　internal　overpressure　and　flame　behavior　within　and　outside　a　1　m(3)　cuboid　test　chamber　with　30　vol%　hydrogen-air　mixtures　were　investigated.　Different　pressure　peaks　and　the　corresponding　mechanisms　were　presented.　Two　regions　of　pressure　oscillation　were　found　in　the　pressure-time　histories.　The　oscillation　frequency　of　one　region　was　independent　of　the　nondimensional　vent　coefficient-　K-V,　and　that　of　the　other　increased　considerably　with　KV.　The　maximum　overpressures　increased　slightly　when　the　vent　sizes　were　large　(K-V　＜=　12.50)　and　increased　significantly　as　the　vent　sizes　were　decreased　(K-V　＞　12.50).　Two　empirical　correlations-Molkov＇s　best-fit　model　and　a　modified　version　of　NFPA　68　(2013)　by　Fakandu　et　al.-　relating　the　vent　sizes　to　overpressure　were　found　to　be　appropriate　for　the　present　study.　A　Mach　disk　was　identified　for　the　smallest　vent　size　(K-V　=　25.00)　in　the　external　flame,　and　a　constant-pressure　jet　flame　was　observed.