We study the Bernstein's polynomial of a family $(W_y)_{y\in Y}$ of hypersurfaces of $\mathbb {C}^n$ with isolated singularities when the family is parametrized by a smooth space $Y$ and defined by an holomorphic mapping $F$ on $\mathbb {C}^n\times Y$. We show that the partition defined by the Bernstein's polynomial $b_y$ of the fiber $W_y$ of the parameter'space $Y$ is locally finite and “constructible”. We prove the existence of a generic Bernstein's polynomial which coïncide with the Bernstein's polynomial of the generic fiber. In the particular case where the family $(W_y)_{y\in Y}$ has a constant Milnor's number we establish the existence of a “good operator in $s$” in the ring of the relative differential operators which are polynomial in $s$, nullifying $F^s$ ; and the existence of a “relative” Bernstein's polynomial.