Abstract The oxygen deficiency, δ, dependence of the second peak effect in the magnetization hysteresis curves was investigated on (La 1− x Sr x ) 2CuO 4−δ single crystals with varius δ at T T c =0.6 . When the magnetic field was applied parallel to the c-axis, the second peak field, B 2 pk , shifted towards higher fields with decreasing δ. The magnetic hysteresis width was suppressed below B 2 pk with decreasing δ, while little δ dependence was detected above B 2 pk . The current dependence of the flux potential barrier evaluated from the magnetic relaxation at T T c =0.7 changed its slope, | ϖU ϖJ | , drastically around B 2 pk . The results indicate a crossover in the creep mechanism around B 2 pk .