An experimental investigation is carried out to investigate the diffusion and mixing in a confined mesoscale channel. A reactor with a rectangular cross section is built. The reactor consists of two horizontal ceramic plates and two vertical quartz windows, forming a rectangular volume of 240×100×6 mm3. By controlling the flow velocity and wall temperature, a flame street that consists of multiple flamelets inside the mixing layer is identified experimentally for the first time. The formation of flame street is due to the process of diffusion and reignition. The number of flamelets in the mixing layer depends on the wall temperature and the flow velocity.