Kraus operators are widely used in describing the evolution of an open quantum system. In this paper, we study the properties of a Kraus operator as a linear combination of unitary matrices and demonstrate that every single Kraus operator can be realized in an interference quantum circuit. We determine the minima of both l 1 and l 0 norm of the combination coefficients, where l 1 norm means the sum of the absolute values of the coefficients and l 0 norm means the number of non-zero coefficients. We find that both of them have clear physical meanings. The l 1 minimum signifies the most constructive interference, and the l 0 minimum provides the simplest way to construct a Kraus operator in a quantum circuit. These results may be useful in understanding interference and Kraus operators as well as in designing a quantum algorithm with a quantum computer in an open environment.