Abstract: 3-D numerical simulation on flow regime has been performed for the equal diameter T-junction with different inlet temperatures for primary and branch pipes by AQUA code to study the stripping and stratification phenomenon. The results show that the inlet branch flow vertically sticks into the primary pipe and only a very narrow channel at the downside of primary pipe is left for the primary flow to pass through. A vortex existing just after this vertical branch flow at the upper side of the primary pipe greatly shrinks the flow area both for the primary flow and the branch flow, thus they are accelerated in this narrow channel. Temperature distribution has been investigated too. The temperature gradient indicates that the mix- ing at the upwind of the branch jet is fierce, but the mixing mainly happens at the downstream of the primary pipe.