Effect of concentration and obstacles on methane-air mixture-deflagration to detonation transition (DDT) was investigated in a long circular duct with lengths of 40 m and inner diameter of 350 mm. Five different concentrations 6%, 8%, 10%, 12% and 14% (in this paper, concentration specifically refers to volume concentration unless otherwise specified) were selected for the various investigations. Experimental results show that flame reaches the maximum velocity when mixtures are near the stoichiometric concentration without obstacles while deflagration to detonation transition (DDT) may occur when obstacles are arranged in the duct. Four kinds of obstacles with blockage ratio of 0.3, 0.45, 0.6 and 0.75 are used in the experiment. The effect of obstacles on the flame velocity was investigate by inserting different number of obstacles (3, 6, 9 and 12) and adjusting spacing of obstacles (175 mm, 350 mm, 525 mm and 700 mm). The blockage ratio of obstacles as well as their spacing and number has great effects on the flame velocity of the mixtures. A high number of obstacles in the duct can increase flame turbulence and lead to flame acceleration. At a mixture volume concentration of 8%, flame propagates faster with an increase in obstacle spacing and DDT could happen. The larger the obstacle blockage ratio, the stronger the interaction of the unburned mixture with a shock wave, which is more beneficial to the acceleration of the flame, while more heat is dissipated with an increase of the obstacle blockage ratio and this is not good for flame acceleration and propagation, so final flame acceleration is determined by the two competing factors. (C) 2016 Published by Elsevier Ltd.