In　this　paper,　the　flame　evolution　and　pressure　dynamics　of　hydrogen-nitrogen-air　explosions　with　nitrogen　addition　ratio　(chi)　ranging　from　0　to　40　%,　ignited　at　three　different　positions　(＂central＂,　＂back＂　or　＂front＂　with　respect　to　the　vent)　in　a　vented　cylindrical　vessel,　were　experimentally　studied.　Experimental　results　reveal　that　the　coupling　effects　of　chi　and　ignition　position　significantly　affect　the　pressure　curves　and　flame　behavior　within　and　outside　the　vessel.　The　higher　the　chi,　the　smoother　the　internal　flame　captured　by　a　high-speed　schlieren　system.　When　chi＜30　%,　the　maximum　reduced　overpressure　(P-max)　at　different　ignitions　decreases　with　increasing　chi,　and　the　central　explosion　yields　the　best　suppression　of　P-max:　when　chi　is　increased　from　0　to　30　%,　P-max　monotonically　decreases　from　232　kPa　to　38　kPa.　However,　the　differences　in　P-max　among　the　three　ignition　positions　become　negligible　when　chi　＞=　30　%.　The　structure　of　the　pressure　peaks　and　the　types　of　oscillations　measured　near　the　vent　depend　on　the　combinations　of　ignition　location　and　chi.　The　formation　of　a　shock　wave　generated　by　the　external　explosion　and　its　effect　on　the　internal　pressure-time　histories　are　described.　In　general,　for　a　given　ignition,　the　maximum　external　overpressure　(Pe-max)　decreases　with　chi　is　increased.　The　most　pronounced　decreasing　trend　of　Pe-max　is　consistently　observed　in　back　explosions　when　chi　ranging　from　0　to　40　%.　Furthermore,　compared　to　other　ignition　positions,　the　highest　P-max　is　always　attained　in　central-ignition　with　chi＜30　%;　while　the　highest　Pe-max　is　always　attained　in　back-ignition　with　chi　＜=　30　%;　as　chi　＞=　10　%,　both　P-max　and　Pe-max　recorded　at　front-ignitions　are　almost　insensitive　to　chi.