Abstract: A series of cyclic biaxial tests on loose sand sunder both drained and undrained conditions with constant amplitudes of strain and stress were performed.The evaluation of micro fabric parameters such as coordinate number,contact orientation and contact forces were studied during the process of failure under cyclic loading.The mechanism of liquefaction and phase transformation was discussed from the viewpoint of micro-mechanism.It was indicated that macroscopic liquefaction was induced due to the cumulative loss of co-ordination number and continuous reduction of contact forces in microscopic fabric.The fundamental reason for sand liquefaction was the incompatibility of fabric and force anisotropies during cyclic loading.The results were valuable for the studies on the micro regularity of sand deformation and the establishment of micro models.