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

摘证明了数域上阶幂等阵相似充条件是它们有相；出了幂等阵相似标准型；讨论了幂等阵线性组合仍是幂等阵充条件。