The necessary and sufficient conditions of similarity of two idempotent matrices are proved, the idempotency of linear combinations of two idempotent matrices is discussed.

摘要证明了数域同阶幂等阵相似的充要条件是它们有相同的秩；出了幂等阵的相似标准型；讨论了幂等阵的合仍是幂等阵的充要条件。