  We study properties of instanton and monopole in an external chromomagnetic
field. Generally, the 't Hooft ansatz is no longer a solution of the Yang-Mills
field equation in the presence of external fields. Therefore, we investigate a
stabilized instanton solution with minimal total Yang-Mills action in a
nontrivial topological sector. With this aim, we consider numerical
minimization of the action with respect to the global color orientation, the
anisotropic scale transformation and the local gauge-like transformation
starting from a simple superposed gauge field of the 't Hooft ansatz and the
external color field. Here, the external color field is, for simplicity, chosen
to be a constant Abelian magnetic field along a certain direction. Then, the
4-dimensional rotational symmetry O(4) of the instanton solution is reduced to
two 2-dimensional rotational symmetries $O(2)\times O(2)$ due to the effect of
a homogeneous external field. In the space $\mib{R}^{3}$ at fixed $t$, we find
a quadrupole deformation of this instanton solution. In the presence of a
magnetic field $\vec{H}$, a prolate deformation occurs along the direction of
$\vec{H}$. Contrastingly, in the presence of an electric field $\vec{E}$ an
oblate deformation occurs along the direction of $\vec{E}$. We further discuss
the local correlation between the instanton and the monopole in the external
field in the maximally Abelian gauge. The external field affects the appearance
of the monopole trajectory around the instanton. In fact, a monopole and
anti-monopole pair appears around the instanton center, and this monopole loop
seems to partially screen the external field.
