To investigate the effect of blockage ratios on the explosion suppression by powder suppressant, an experimental study was performed to suppress the methane-air explosion in a 5L duct with different blockage ratios and various concentrations of BC dry powder. The results indicate that flames experienced both the spherical and finger-shaped stages. Furthermore, the smoothness of flame front initially decreased and then increased. Flame propagation velocities were higher with larger blockage ratios except for phi = 1. The maximum peak over-pressure (MPP) with the blockage ratio was slightly increased till phi reached 0.7 then surged sharply. The MPP decreased as the powder concentration increased. The maximum drop rate in the MPP being 34.8%-59.9%, depending on powder concentrations, occurred at the blockage ratio between 0.4 and 0.6. The result is ascribed to the competition between the suppression augmentation by the higher venting-generated turbulence and the suppression attenuation by the shorter residence time of the particle. However, the drop rate was relatively less promoted by increasing the concentration from 80 g/m(3) to 240 g/m(3). The inhibitor at higher concentration was less effective. An inhibition mechanism is explained by analogy to droplet group combustion, in which the decomposition regime of NaHCO3 differs at different concentrations.