  In this article we study the behaviour of semistable principal $G$-bundles
over a smooth projective variety $X$ under the extension of structure groups in
positive characteristic. We extend some results of Ramanan-Ramanathan
\cite{Ramanan-Ramanathan} on rationality of instability flags and show that the
associated vector bundles via representations of $G$ are not too unstable and
the instability can be bounded by a constant independent of semistable bundles.
As a consequence of this the boundedness of the set of isomorphism classes of
semistable $G$-bundles with fixed degree and Chern classes is proven.
