We have studied an anomalous microwave (mw) response of superconducting YBa2Cu3O 7−δ (YBCO) microstrip resonators in the presence of a weak dc magnetic field, H dc . The surface resistance (Rs) and reactance (Xs) show a correlated non-monotonic behaviour as a function of H dc . Rs and Xs were found to initially decrease with elevated H dc and then increase after H dc reaches a crossover field, Hc, which is independent of the amplitude and frequency of the input mw signal within the measurements. The frequency dependence of Rs is almost linear at fixed H dc with different magnitudes (Hc). The impedance plane analysis demonstrates that rH, which is defined as the ratio of the change in Rs(H dc ) and that in Xs(H dc ), is about 0.6 at H dc Hc. The H dc dependence of the surface impedance is qualitatively independent of the orientation of H dc .