The purpose of this study was to investigate the relationships between force fluctuation during isometric plantar flexion and the sustainable time for single-leg standing. Fourteen healthy males (21±1 years) performed unilateral (preferred leg) force matching tasks and single-leg quiet standing. Force matching tasks were performed to maintain isometric plantar flexion for 15 s at levels corresponding to 10% and 20% maximal voluntary contraction (MVC) with the visual feedback of force. Force fluctuation during force matching tasks was quantified as the standard deviation of force. Sustainable time for single-leg quiet standing was performed to maintain a single-leg quiet standing barefoot on a platform using the preferred leg with their eyes closed. Force fluctuation was significantly greater in 20% MVC task compared to 10% MVC task. The sustainable time for single-leg quiet standing was strongly correlated with force fluctuation in 20% MVC task (r=−0.56, p=0.04). However, it was not related to force fluctuation in 10% MVC task (r=0.19, p=0.52) or MVC value (r=0.13, p=0.65). These results suggest that a specificity of contraction intensity is observed between force steadiness and the posture stability during single-leg quiet standing; force steadiness during 20% MVC plantar flexion is one of the important components for posture stability during single-leg quiet standing.