We have measured the magnetostriction on single crystals of high- T c superconductors. Substantial relaxation in magnetostriction was found and could be related to the flux creep phenomenon. The qualitative behavior of the relaxation curves at 5 K was consistent with the analysis based on the pinning-induced magnetostriction model. The results demonstrate magnetostriction measurement to be a potential means to investigate not only the pinning effect but also flux creep kinetics in these compounds.