With　higher　penetration　of　renewable　energy　sources　(RES),　more　combined-cycle　gas　turbines　(CCGTs)　will　be　deployed.　These　flexible　gas-fired　generators　can　provide　efficient　system　reserve　to　deal　with　rapid　power　fluctuation　of　renewable　energy.　Therefore,　an　accurate　reserve　allocation　(RA)　on　CCGTs　is　necessary　to　guarantee　efficiency　and　safety　of　integrated　electricity　and　natural　gas　system　operation.　Traditional　RA　model　ignores　the　non-dispatchable　start-up　and　shut-down　processes　of　combustion　turbines　and　steam　turbines　(i.e.　transition　trajectories)　in　CCGTs　and　is　less　considered　about　the　effect　from　natural　gas　system.　A　novel　RA　model　is　proposed　considering　the　effect　of　transition　trajectories　of　CCGTs,　gas-electricity　coupling　and　unit　commitment　to　meet　the　demand　of　load　and　RES　forecast　error　and　N-1　contingencies.　The　simulation　results　show　that　transition　trajectories　and　gas　load　level　greatly　affect　the　reserve　scheduling　of　CCGTs.　©　2019　IEEE.