  We extend Thurston's combinatorial criterion for postcritically finite
rational maps to a class of rational maps with bounded type Siegel disks. The
combinatorial characterization of this class of Siegel rational maps plays a
special role in the study of general Siegel rational maps. As one of the
applications, we prove that for any quadratic rational map with a bounded type
Siegel disk, the boundary of the Siegel disk is a quasi-circle which passes
through one or both of the critical points.
