  We consider the twistor space ${\cal P}^6\cong{\mathbb R}^4{\times}{\mathbb
C}P^1$ of ${\mathbb R}^4$ with a non-integrable almost complex structure ${\cal
J}$ such that the canonical bundle of the almost complex manifold $({\cal P}^6,
{\cal J})$ is trivial. It is shown that ${\cal J}$-holomorphic Chern-Simons
theory on a real $(6|2)$-dimensional graded extension ${\cal P}^{6|2}$ of the
twistor space ${\cal P}^6$ is equivalent to self-dual Yang-Mills theory on
Euclidean space ${\mathbb R}^4$ with Lorentz invariant action. It is also shown
that adding a local term to a Chern-Simons-type action on ${\cal P}^{6|2}$, one
can extend it to a twistor action describing full Yang-Mills theory.
