Lockout/Tagout is an effective program to protect plant workers from exposure to hazardous energy, sources. Because of the complexity, of a process plant, it is a time-consuming job for maintenance managers to achieve certain Lockout/Tagout plans for Turnaround Maintenance. Process flow and layout information are essential to this planning, so Computer-Aided Design (CAD) systems are becoming very useful resources. In this study, we propose a computer-aided method for planning Lockout/Tagout programs in process plants. To isolate certain equipment, minimum required operations are explored and reported automatically,. Algorithms that investigate CAD data are shown in order to determine isolated areas. Data structures which mitigate limitation of calculation time are applied for large-scale Plants. We also consider the integration of isolated areas and the elimination of overlapping operations.