Summary: The paper researches the rank of the combinations of two idempotent matrices \(P\) and \(Q\), i.e., the rank of \(aP+bQ-cQP\) (where \(a,b,c\in \mathbb{C}\), \(a\neq 0,b\neq 0\)). By using the properties of the nullspace of the matrix and isomorphisms of the linear space, we get that the rank of \(aP+bQ-cPQ\) is a constant and is equal to the rank of \(P-Q\) when \(c=a+b\), elsewise equal to the rank of \(P+Q\) when \(c \neq a+b\), which generalize the results of J. J. Koliha and V. Rakočević [Linear Algebra Appl. 418, No. 1, 11–14 (2006; Zbl 1104.15001)].