  In this paper, we generalize the unitarity method to two-loop diagrams and
use it to discuss the integral bases of reduction. To test out method, we focus
on the four-point double-box diagram as well as its related daughter diagrams,
i.e., the double-triangle diagram and the triangle-box diagram. For later two
kinds of diagrams, we have given complete analytical results in general
(4-2\eps)-dimension.
