  We calculate the Bose-Einstein condensation (BEC) temperature of spin-1
atomic bosons in external magnetic field, taking into account the influence of
the quadratic Zeeman effect. In case that the quadratic Zeeman coefficient is
positive, the BEC temperature exhibits a nontrivial dependance on the magnetic
field and a magnetic-field-induced reentrant phenomenon of BEC is observed.
This phenomenon could be well understood by the competition between the linear
and quadratic Zeeman effects. Reentrance of BEC in a trapped spinor Bose gas is
also discussed.
