Abstract: Researches ontwo-phase flow and pool boiling heat transfer in microgravity,which included ground-based tests, flight experiments, andtheoretical analyses, were conducted in the National MicrogravityLaboratory/CAS. A semi-theoretical Weber number model was proposedto predict the slug-to-annular flow transition of two-phasegas-liquid flows in microgravity, while the influence of theinitial bubble size on the bubble-to-slug flow transition wasinvestigated numerically using the Monte Carlo method. Two-phaseflow pattern maps in microgravity were obtained in the experimentsboth aboard the Russian space station Mir and aboard IL-76 reducedgravity airplane. Mini-scale modeling was also used to simulatethe behavior of microgravity two-phase flow on the ground.Pressure drops of two-phase flow in microgravity were alsomeasured experimentally and correlated successfully based on itscharacteristics. Two space experiments on pool boiling phenomenain microgravity were performed aboard the Chinese recoverablesatellites. Steady pool boiling of R113 on a thin wire with atemperature-controlled heating method was studied aboard RS-22,while quasi-steady pool boiling of FC-72 on a plate was studiedaboard SJ-8. Ground-based experiments were also performed both innormal gravity and in short-term microgravity in the drop towerBeijing. Only slight enhancement of heat transfer was observed inthe wire case, while enhancement in low heat flux anddeterioration in high heat flux were observed in the plate case.Lateral motions of vapor bubbles were observed before theirdeparture in microgravity. The relationship between bubblebehavior and heat transfer on plate was analyzed. Asemi-theoretical model was also proposed for predicting the bubbledeparture diameter during pool boiling on wires. The resultsobtained here are intended to become a powerful aid for furtherinvestigation in the present discipline and development oftwo-phase systems for space applications.