Flux creep in high Tc oxide YBa2Cu3Oy superconductor in critical state has been studied by observing variation of magnetization with time. Experimental results show that for the zero field cooling sample, the magnetization decays with time following the law M∝lnt over the observation time period, and this process accompanies by the occurrence of flux jumps. The rate of the magnetization decay dM/dlnt, as a function of the field, exhibits a maximum near H=3HP. It is believed to be related to weak link superconductivity. dM/dlnt decreases monoto-neously with the rising temperature in the temperature range above 77K. Finally, a discussion is made about the activation energy of the flux lines U0.