Fluid flow at the T-junction of a model of a supercritical water microreactor is studied experimentally and numerically. Experimental cells are fabricated from three pieces of sapphire, the flow path width is 2 min while the depth is 2.5 mm. SC/Tetra Is used as the computational fluid dynamics software and three-dimensional calculation is performed. Experimental results and numerical results ore compared for the condition where the temperature and pressure of hot water are 389 K and 45 MPa, respectively. Natural convection Is found to be dominant even In such a small now area.