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Abstract Rock friction tests have made profound contributions to our understanding of earthquake processes. Most rock friction tests focused on fault strength evolution during velocity steps or at specific slip rates and the characteristics during stick‐slip events such as dynamic rupture propagation and the transition from stable sliding to instability, with little attention paid to the transient acceleration and deceleration periods. Here, we present Westerly Granite fault friction test results using a unique pneumatically powered apparatus with high acceleration of up to 50 g, focusing on the transient stages of fast fault acceleration and deceleration during both high‐speed sliding and stick‐slip events. Our data demonstrates the dominating velocity‐weakening behavior at transient stages of fault acceleration and deceleration, with a 1/V dependence for peak friction and deceleration lobe consistent with the flash‐heating model but with the acceleration lobe consistently deviating from the 1/V dependence. Our analysis of velocity‐dependent friction between dynamic rupture events, stick‐slips, and high‐speed friction tests reveals the significance of high acceleration in influencing transient fault weakening during dynamic weakening. We further demonstrate that the deviation of the friction‐velocity curve from the 1/V trend during fault acceleration is associated with the contribution of the dynamic rupturing process during the initiation of fault slip.