Phase structure and phase transitions in dense QCD are studied using the Schwinger-Dyson (SD) method in the improved ladder approximation.
Due to strong coupling effects at low densities, such as the of
f-Fermi surface degrees of freedom and antiquarks contribution to the pairing gap owing to the large effective coupling, the gap, critical temperature
and those ratio deviate from those weak coupling values at low 
density.
Nevertheless, the ratio of the physical quantity for the CFL and that
for the 2SC does not differ so much from the weak coupling value. As a
consequence, the CFL state always dominates over 2SC state and the
critical temperatures to quark-gluon plasma (QGP) from both states
coincides in the chiral limit. The energy gain in the CFL state relative
to the 2SC state gets smaller towards the critical line dividing $(\mu,T)$
plane into the QGP and CFL phases, and thus small perturbations can remove
the degeneracy of these critical lines. As one of such perturbations, the
effect of the strange quark mass $m_s$ on the quark-pairing is examined.
In particular, using a simple kinematical criterion, we discuss  the
behaviour of the ``{\it color-flavor unlocking line}'' on which the CFL
phase turns into the 2SC phase, against the variation of $m_s$.