Ten different formulas of possibility degree are analyzed. It is pointed out that such a ranking method for interval numbers is unreasonable by counterexamples from two different viewpoints. The relationship between possibility degree matrix and fuzzy judgment matrix is analyzed, and the reason leading to the unreasonable ranking result is investigated. Finally, by utilizing the possibility degree matrix, the Boolean matrix is established, and an improved algorithm of ranking interval numbers based on the Boolean matrix is proposed. The rationality of the proposed approach is proved theoretically.