In this study, the flow features of vortex shedding from a circular cylinder oscillating along the direction of the flow and its characteristics of fluid force were simulated using a vortex method at the frequency ratios f/f_K=1.6 and f/f_K=3.0. the ranges of amplitude ratio 2a/d=0〜0.75 and Reynolds number Re=530. As a result of calculations, two typical flow patterns of the lock-in were shown, and it was confirmed that the calculated flow patterns were in reasonable agreement with previous experimental results. The fluid force act on the oscillating cylinder was investigated. It is understood that the drag amplitude is increased according to the oscillating condition and there is a part that becomes an impellent in the target momentarily. Moreover, it was clarified that the amplitude of the lift coefficient was larger than the amplitude of the drag coefficient in the lock-in of f_<VK>f=1/2, and the amplitude of the drag coefficient was larger than the amplitude of the lift coefficient in the lock-in of f_VK/f=1/1. When the amplitude ratio 2a/d grows, this tendency becomes remarkable.