System failure due to a progressive defect in memory cell-array of DRAM was studied with automated test equipment. In order to find out relationship correctable single-bit fault and system failure, memory cells with single-bit fault by a cross-defect were selected. After high voltage and temperature stress, a soft cross-defect was changed into a hard cross-defect. Consequentially, invalid operation by a degraded cross-defect causes array-failure. Based on the failure analysis, methods to prevent array-failure are proposed, and applied to DRAM successfully.