Abstract: The problem of searching all feedback loops of a system dynamics model is equal to calculating all essential circuits of a corresponding simple directed graph. The keyof the problem is time complexity. A newconcept named strong kernel was defined, and the algorithm based on strong kernel for searching all essential circuits of a simple directed graph was proposed. An illustrative example was presented and the complexity of the algorithmwas analyzed. In terms of time complexity, the proposed algorithm is superior to that of the determinant algorithm based on the concept of kernel of directed graphs.