  If the annihilation products of dark matter (DM) are non-relativistic and
couples directly to a light force mediator, the non-perturbation effect like
final state bound state (FBS) formation and final state Sommerfeld (FSS) effect
must be considered. Non-relativistic region of final particles will appear when
there is small mass split between DM and products, so we study those effects in
the degenerate region of mass (including kinematics forbidden case) using two
specific models. We demonstrate that FBS effect will significantly modify the
DM relic abundance comparing to the standard perturbation calculation in some
mass split region. We emphasize that FBS effect is comparable to the FSS effect
in those mass split. The conservation angular momentum are subtle considering
FBS formation, in some cases there may be not $s$-wave, so we use two models
exhibit the different partial wave FBS effect contribution. We also show that
the FBS formation with vector boson emission process also contributes in DM
relic abundance, and first calculate the $p$-wave FSS effect in the specific
model.
