The paper presents an approach to performance analysis of heterogeneous parallel algorithms. As a typical heterogeneous parallel algorithm is just a modification of some homogeneous one, the idea is to compare the heterogeneous algorithm with its homogeneous prototype, and to assess the heterogeneous modification rather than analyse the algorithm as an isolated entity. A criterion of optimality of heterogeneous parallel algorithms is suggested. A parallel algorithm of matrix multiplication on heterogeneous clusters is used to illustrate the proposed approach.