We have measured magnetic relaxation M versus time in a twin-free single crystal of YBa2Cu3Ox for magnetic fields applied parallel to a-b planes. Magnetic relaxation is studied as a function of temperature (T) for a fixed field (H) and at a fixed T as a function of H. Long-time M versus t corves were mainly obtained for the partial field penetrating regime. All log M versus log t curves obtained in the partial-field regime present a kink at time t* separating two distinct regions. The kink gradually disappears as the applied field fully penetrates the sample. The value of t* is constant for ail field values, Similar but more accentuated differences are observed when the effective activation energy U extracted from the data is plotted against M(t). A linear behaviour of U with M is observed in the full field penetrated regime. We estimate that the Bean penetration depth increases exponentially with H in the partial-field regime. We also determined the range of temperatures for which surface barriers are important at the initial stage of relaxation.105284289