An experimental study was conducted on turbulent flow and mixing in a counter-flow type T-junction. We measured the velocity and concentration fields simultaneously by combining PIV and PLIF. Special attention was directed to the concentration fluctuation near the channel wall, which might bring about the high-cycle thermal fatigue in case of mixing of hot and cold flows. The velocity ratio of the counter-channel flow to the main-channel flow was changed from 1.0 to 5.0. The fluorescent dye was mixed in the main-channel flow. The dominant structures of the fluctuating velocity and concentration fields that cause the concentration fluctuation near the channel wall were analyzed by POD. It was found that the concentration fluctuation near the channel wall was caused by the superposition of the spanwise wobbling motion of the mixing interface of two flows and the rotational oscillation of the flows.