The low field microwave absorption in Electron Paramagnetic Resonance (EPR) of superconducting $YBa_2Cu_3-O_7$ has been attributed to the flux slippage at the Josephson junction, however, whose whereabouts has not been exactly singled out yet. To investigate the low field absorption behavior, sintered and/ crushed polycrystalline powders of mean particle diameter 2.73 and 5.6 μm and a flux grown single crystal of size 0.5 mm were prepared. The low field EPR absorption intensity is much stronger in the polycrystalline samples of size 5.6 μm than in the rest. From the consideration that the grain boundary area of a polycrystalline 5.6 μm specimen is largest while that of the single crystalline specimen is smallest, it is quite reasonable to deduce that the EPR peak intensity is largest in the 5.6 μm specimen if the EPR spectrum is ascribed to the flux slippage at the grain boundary. Notwithstanding the absence of the grain boundaries in the single crystalline specimen, the presence of the low field EPR absorption tends to indicate that the microtwins in the high Tc superconductor also act as a network of Josephson junctions. From the frustration phenomena of weak coupled grains, superconducting cluster size has been estimated in the polycrystalline and single crystalline specimen. Also, by the variation of modulation amplitude, relaxation effect of this clusters can be observed.
