Abstract
In the superstring models we have not only the complete {\bf 27} multiplets of $E_6$ but also extra incomplete $({\bf 27}+{\overline {\bf 27}})$ chiral supermultiplets being alive at low energies. Associated with these additional multiplets, when the gauge symmetry contains more than one $U(1)$ gauge group, there may exist gauge kinetic mixings among these $U(1)$ gauge groups. In such cases the effect of gauge kinetic mixings should be incorporated into the study of unification of gauge couplings. We study these interesting effects systematically in these models. The string threshold effect is also taken into account. It is found that in the four-generation models we do not have a advisable solution of string unification of gauge couplings consistent with experimental values at the electroweak scale. We also discuss the possible scenarios to solve this problem.
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