Using the Dadarlat isomorphism, we give a characterization for the Kasparov product of ${{C}^{*}}$-algebra extensions. A certain relation between $KK\left( A,\,Q\left( B \right) \right)$ and $KK\left( A,\,Q\left( KB \right) \right)$ is also considered when $B$ is not stable, and it is proved that $KK\left( A,\,Q\left( B \right) \right)$ and $KK\left( A,\,Q\left( KB \right) \right)$ are not isomorphic in general.