  We explore for the first time effects of the magnetic field on the escape of
$^{22}$Na positrons and on the flux evolution of annihilation 511 keV line in
novae. It is shown that for the white dwarf magnetic field of $\sim 10^6$ G the
field of the expanding nova shell is able to significantly impede positrons
escape and increase the time of the nova emission in 511keV up to hundreds
days.
