The　research　covers　the　experimental　investigation　of　two-phase　flow　instabilities　in　parallel　vertical　internally　ribbed　pipes　at　low　mass　flow　rate　in　the　high-pressure　steam-water　test　loop.　The　tested　tube　is　an　outlet　diameter　of　28.6mm　and　a　wall　thinkness　of　5.8mm.　The　pressure-drop　type　oscillations　are　found　in　the　experiments.　The　effects　of　pressure,　mass　flow　rate　and　inlet　subcooling　on　instabilities　are　studied.　The　results　show　that　the　boundary　heat　flux　of　pressure-drop　type　oscillations　increase　with　increase　of　inlet　pressure,　mass　flow　rate　and　inlet　subcooling.　It　is　found　that　sysetem　pressure,　inlet　subcooling　and　mass　flow　rate　have　great　effects　on　amplitudes　and　periods　of　two-phase　flow　instabilities.　In　addition,　it　is　found　that　the　effects　of　pressure　on　periods　of　pressure-drop　type　oscillations　represent　two　different　trends　in　the　test.　When　pressure　is　lower　then　4Mpa,　the　periods　of　pressure-drop　type　oscillation　increase　with　pressure　increasing,　but　when　pressure　is　higher　then　4Mpa,　the　oscillation　periods　decrease　with　pressure　increasing.