For　a　better　design　of　1000　MW　ultra　supercritical　pressure　boiler　with　spirally　water　wall　tubes,　an　experimental　study　has　been　performed　in　the　State　Key　Laboratory　of　Multiphase　Flow　in　Power　Engineering,　Xi＇an　Jiaotong　University　to　investigate　the　instability　of　the　steam-water　two-phase　flow　inside　a　smooth　tube　with　an　inclined　angle　of　20　degree　from　the　horizon.　The　static　hydrodynamic　characteristic　curves　are　obtained　and　the　dynamic　instabilities　are　also　analyzed.　The　boundary　conditions　of　the　pressure　drop　oscillation　and　the　density　wave　oscillation　are　proposed.　The　effects　of　pressure,　mass　flux,　inlet　and　outlet　throttle,　inlet　subcooling　and　the　compressible　volumes　are　analyzed　primarily.　The　results　demonstrate　that　the　above　parameters　significantly　influence　the　critical　heat　flux　for　the　mentioned　two　types　of　oscillations　and　the　density　wave　oscillations　are　the　major　type　of　oscillations　in　boilers　and　steam　generators.　The　density　wave　oscillations　occur　at　a　higher　heat　flux　than　the　pressure　drop　oscillations　with　the　other　operating　parameters　remaining　constant.　The　inducing　mechanisms　of　the　above　two　types　of　oscillations　are　clarified.　The　variation　ranges　of　critical　heat　flux　and　critical　vapor　quality　are　illustrated.　Two　empirical　correlations　predicting　the　onset　of　pressure　drop　oscillation　and　density　wave　oscillation　are　derived　from　the　experimental　data.　(C)　2016　Elsevier　Inc.　All　rights　reserved.