Critical currents of YBaCuO ceramics were measured at 77 K by transport measurements in an applied magnetic fieldH. A hysteretic behavior between 10 and 10 000 G was observed. For decreasing fields, the critical currentJcobeys a power lawH−nwithnranging between 0.6 and 2.8. The exponentnis correlated with the microstructure and the average grain size of the ceramic. This power law behavior is discussed in terms of modified flux creep and flux flow regimes related to the microstructure.