Addressing the requirement for defect detection, this paper proposes critical operation oriented dynamic symbolic execution. First, based on the defined critical operations and the relevant critical paths, a set of initial inputs are evaluated by computing the ability of covering critical operations under different contexts, and efficient initial inputs can be selected for the following dynamic symbolic execution. Second, leveraging the critical operations, dynamic symbolic execution is guided to explore paths which are more prone to defects. In this way, defect detection becomes a process of locating critical operations and exploring critical paths. A prototype system called CrashFinder is implemented and tested on a number of Linux x86 executables. The experimental results show that this approach is effective in initial input evaluation and efficient in defect detection.