FIM (Flow Induced Motion) is a kind of widespread and high-energy phenomenon. In application, FIM could be used to harvest hydrokinetic energy from ocean/river currents. In this study, a spring-mounted rectangular cylinder were numerically investigated in a flow domain for the ranges of 7500 < Reynolds number <187500 (0.1 m/s < flow speed <2.5 m/s) to examine the effect of its cross-sectional aspect ratio on the FIM responses and hydrokinetic energy conversion. Results indicate that in general, high aspect ratio has a negative effect on FIM. When the aspect ratio increases from 1/6 to 1.5, the FIM amplitude is gradually suppressed. When the aspect ratio reaches up to 2.0, both VIV (Vortex Induced Vibration) and galloping won't occur at any flow speed. For energy harvesting, when the aspect ratio decreases from 2 to 1/4, the converted power shows the increasing trend. The maximum FIM energy conversion efficiency also accordingly increases to 15.5%. It should be noted that when the aspect ratio reduces to 1/6, the maximum power keeps the increasing trend; however, the total energy conversion efficiency is not further enhanced, which implies the optimal aspect ratio for energy harvesting is around 1/4. (C) 2018 Elsevier Ltd. All rights reserved.