  The Kondo effect in coupled quantum dots is investigated theoretically under
magnetic fields. We show that the magnetoconductance (MC) illustrates peak
structures of the Kondo resonant spectra. When the dot-dot tunneling coupling
$V_C$ is smaller than the dot-lead coupling $\Delta$ (level broadening), the
Kondo resonant levels appear at the Fermi level ($E_F$). The Zeeman splitting
of the levels weakens the Kondo effect, which results in a negative MC. When
$V_{C}$ is larger than $\Delta$, the Kondo resonances form bonding and
anti-bonding levels, located below and above $E_F$, respectively. We observe a
positive MC since the Zeeman splitting increases the overlap between the levels
at $E_F$. In the presence of the antiferromagnetic spin coupling between the
dots, the sign of MC can change as a function of the gate voltage.
