  In this paper, we investigate thermal entanglement in a
superconducting-quantum-interference-device qubit coupled to a cavity field. We
show that the entanglement can be manipulated by varying temperature and an
effective controlling parameter $B$ which depends on the external field and
characteristic parameters of the system. We find that there exists a critical
value of the controlling parameter $B_c$. Under a fixed temperature, increasing
$B$ can increase entanglement in the regime of $B<B_c$, while the entanglement
decreases with increasing $B$ in the regime of $B>B_c$.
