Dual-cooled annular fuel is being developed in KAERI. Dual-cooled annular fuel has small gaps between fuel rods compared to circular fuel rod. Because of this, a pulsation occurred between subchannels. In this paper, thermal mixing by pulsation occurred at the between subchannels in parallel flow to single rod in square channel has been performed through temperature measurement. With decreased of the distance between rod and wall, the thermal mixing effect was increased. In addition, the thermal mixing effect at subchannel was increased by mixing vane