Let A and U be Banach algebras such that U is also a Banach Abimodule with compatible algebra operations, module actions and norm. By defining an approprite action, we turn l 1 -direct product A × U into a Banach algebra such that A is closed subalgebra and U is a closed ideal of it. This algebra, is in fact semidirect product of A and U which we denote it by A ⋉ U and every semidirect products of Banach algebras can be represented as this form. In this paper we consider the Banach algebra A ⋉ U as mentioned and study the derivations on it. In fact we consider the automatic continuity of the derivations on A ⋉ U and obtain some results in this context and study its relation with the automatic continuity of the derivations on A and U . Also we calculate the first cohomology group of A ⋉ U in some different cases and establish relations among the first cohomology group of A ⋉ U and those of A and U . As applications of these contents, we present various results about the automatic continuity of derivations and the first cohomology group of direct products of Banach algebras, module extension Banach algebras and θ-Lau products of Banach algebras. MSC(2010): 16E40; 46H40; 46H25.