  We consider the thermal production of axino dark matter in high-scale
supersymmetry where all the superpartners except the axino are heavier than the
maximum and reheating temperatures. In this case, the axinos are produced
dominantly in pairs from the scattering of SM particles in thermal plasma in
the early Universe after inflation. We find that the thermal averaged
scattering cross section for the axino pair production is given by
$\langle\sigma v\rangle \propto T^4$ in Kim-Shifman-Vainstein-Zakharov (KSVZ)
axion model, while it does not depend on the temperature in
Dine-Fischler-Srednicki-Zhitnitski (DFSZ) axion model. As a result, the axinos
produced during the early matter domination is diluted by the entropy
production, so the axino abundance is determined mainly by the reheating
temperature, unlike the case with gravitino dark matter. We show that the axino
pair production in DFSZ model opens up new parameter space for axino dark
matter, due to non-decoupled Higgsino interactions at tree level.
