  A new method for calculation of the mixing angle between the hadrons within
QCD sum rules is proposed. In this method, the mixing is expressed in terms of
quark and gluon degrees of freedom. As an application, the detailed calculation
of the mixing angle between heavy cascade baryons Xi_Q and Xi_Q', Q=c,~b is
presented and it is found that the mixing angle between Xi_b (Xi_c) and Xi_b'
(Xi_c') is given by theta_b = 6.4^\circ \pm 1.8 ^\circ (theta_c = 5.5^\circ \pm
1.8^\circ).
