The fluctuating and time-averaged aerodynamic forces acting on two circular cylinders and characteristics of flow over the cylinders in tandem arrangement were investigated experimentally at a Reynolds number of 6.5 × 10⁴. The position of the downstream cylinder was varied systematically up to the spacing of 9 diameters and time-averaged drag, fluctuating pressure, fluctuating lift and drag forces acting on the cylinders were measured. Two peaks in each of the distribution of the fluctuating lift and drag forces acting on the downstream cylinder were observed when the cylinders were placed before the critical spacing. The value of the fluctuating lift and drag forces at the larger peak that occurred at a spacing ratio of 1.4 was about 2 and 2.8 times, respectively that of a single cylinder. The magnitude of the fluctuating lift and drag forces acting on the downstream cylinder was related very sensitively with the reattachment position of the separated shear layers from the upstream cylinder onto the downstream one and the fluctuating lift and drag forces became maximum when the reattachment position of the shear layers reached minimum and vice versa.