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

摘要证明了数域上两个同阶幂等阵相的充要件它们有相同的秩；出了幂等阵的相型；讨论了两个幂等阵的线性组合仍幂等阵的充要件。