The paper discusses the design and implementation techniques for a PARLOGInteractive tracing debugger.The authors modlfythe L.Byrd’s Box model and make It ca.pable to illustrate the sequential execution model of PARLOG.In sequen tial executlormodel,It Is easy for programmer to debug program because the computation Is determin.lsm.The paper Introduces three aspects of the research Indebugger implementation Indetall:define the debugging level;change flat computation In sequentlal model to treeshape computation;complete tracing algorithms in different level.