In this study, the flow features of vortex shedding from a pair of tandem arranged circular cylinders oscillating along the direction of the flow were observed by visualizing water flow experiment at the ranges of the frequency ratio f/fK=0～6.2 (28 steps), the amplitude ratio 2a/d=0.25, 0.5, 0.75 and 1.0, the distance ratio L/d=1.5, 2.5 and 5.5 and Reynolds number Re=640. The variations of mean vortex shedding frequency were investigated. Although the cylinder oscillation frequency f is lower than the natural Karman vortex's frequency fK, the lock-in phenomenon can be seen even in tandem arrangement case. The lock-in ranges have been shown in each distance ratio L/d. It is found that the lock-in ranges become wide when comparing with the single cylinder case. The typical flow patterns of the lock-in or un-lock-in states were shown every distance ratio L/d. In distance ratios L/d=2.5 and 5.5, the twin vortex street from the 2nd cylinder (rear set cylinder) was not seen. The 2nd circular cylinder has more flow patterns than the 1st circular cylinder. The flow pattern of the 2nd circular cylinder was complicated, and various.