The hydrodynamic interference between two circular cylinders in tandem and side by side arrangements was investigated by measuring of lift and drag on each cylinder. The time variations of interference lift and drag coefficients in each arrangement were observed at center-to-center pitch ratios of P/D=1.25 and 2.5 and Reynolds number of $Re=1.5 imes10^4$. Average interference lift and drag coefficients were also observed at pitch ratios from P/D=1.25 to 2.5 and Reynolds number from $Re=1.5 imes10^4$ to $1.5 imes10^4$. The hydrodynamic interference between two circular cylinders differed with the shape of the arrangement and the pitch ratio, but the characteristics were revealed by measuring of lift and drag on each cylinder.