The purpose of this work is to understand the thermal-hydraulic characteristics of the closed two-phase thermosiphon loop. To do this, the experimental apparatus has been designed and constructed. It consists of evaporator part, condenser part, gas line, liquid line and recirculation line, and each part is made of copper tube with 17 mm ID and 19.05 mm OD. The length of Evaporator and condenser is 200 cm. Especially, the evaporator and the condenser parts are annular tubes whose inner tubes are heated or cooled by water flowing through annular passages. In this experiment, water is used as the working fluid in thermosiphon loop. We have observed the typical temperature and heat flux data. And we have also observed flow instabilities. Flow instabilities are presented by transient temperature, pressure, main flow rate data. There have been two kinds of instability - density wave oscillation and flow excursion. Local heat fluxes and wall temperatures along the evaporator and the condenser have been measured using the RdF micro foil heat flux sensors.