As　the　leakage　rate　is　one　of　the　important　factors　of　natural　gas　pipeline　operation　that　have　not　been　well　defined,　it　is　necessary　and　important　to　give　an　analytical　equation　for　the　leakage　flow　rate　of　natural　gas　pipelines　to　improve　the　accuracy　of　simulation　and　estimation　of　leakage　accidents.　Based　on　the　theories　of　thermodynamics　and　fluid　mechanics,　together　with　the　use　of　ideal　gas　equation　of　state,　adiabatic　equation　and　energy　conservation　equation,　the　natural　gas　leakage　process　was　studied　and　divided　into　two　leakage　phases,　the　critical　phase　and　the　sub-critical　phase.　Equations　were　obtained　for　calculating　the　leakage　flow　rate　of　the　whole　leakage　process,　and　examples　were　given　to　analyze　the　influences　of　various　parameters　on　leakage　flow　rate.