Abstract: Random test generation technology is one of the most important methods for the verification of modern VLSI．Coverage directed test generation is one of the hot topics in this area,it automates the feedback from coverage analysis to test generation to increase the efficiency and quality of the verification process and reduce the time and effort needed to implement a verification plan．So far,considerable efforts have been invested in coverage directed test generation to find good ways to automate the process of effective bias generation．In this paper,we give a deep discussion and analysis the research and practice of coverage directed test generation technology．We also give our remarks and advices on different methods in coverage directed test generation technology．