An experimental study has been done to investigate the explosion characteristics of aluminum powders with different sizes and concentrations in a 20 L spherical explosion vessel. Two different sizes of aluminum powder were used : $15.1{mu}m$ and $34.8{mu}m$ with a volume mean diameter. The results revealed that $15.1{mu}m$ Al powder has a Lower explosion limit (LEL) of $40g/m^3$, a maximun explosion pressure ($P_{max}$) of 9.8 bar and a maximum rate of pressure rise ($[dP/dt]_{max}$) of 1852 bar/s, in $34.8{mu}m$ Al powder, LEL of $70g/m^3$, $P_{max}$ of 7.9 bar and $[dP/dt]_{max}$ of 322 bar/s. The LEL of Al powders tended to increase with the increase of particle size. Also, it was found that the flame velocity calculated from the powder with $15.1{mu}m$ was about 5 times higher than that of the powder of $34.8{mu}m$.