The effects of channel width, wall temperature, and flow rate on the flame regimes and propagation speeds of non-premix flames in a mesoscale combustor were studied in methane and propane-air-oxygen mixing layers. A flame street structure which consists of multiple triple flamelets in the mixing layer of the reactants was observed experimentally. Depending on the flow rate, it was found that there are two different flame regimes, an unsteady bimodal flame regime and a flame street regime where there are multiple stable triple flamelets. It was found that the separation distance of the flamelets increased due to the dilution effect of the products and that the size of the flamelets was proportional to the width of the mixing layer. A scale analytical model was developed to qualitatively explain the mechanism of flame streets. The propagation speed of the mesocale non-premixed flame cell was measured. It was found that the edge flame speed was significantly faster than that the conventional triple flame. The effects of wall heat loss and friction were discussed. A model of pressure gradient assisted flame propagation was proposed.