I think I confused with the concept of elementary class and abstract elementary class.
We see in the definition of AEC that each elementary class is an AEC.
Let $l=\{\le\}$, $T=\emptyset$, $K=mod(T)$. $(K,\prec)$ is an elementary class. Take $[0,1] ,[1,2],[0,3] \in K$. $[0,1]\cong[1,2]$ and $[0,1]\cong [0,3]$ and $[0,1]\prec[0,1]$ but $[1,2]$ is not an elementary substructure of $[0,3]$ .
So $(K,\prec)$ is an elementary class which is not abstract elementary class (since it not closed under isomorphism). am I write? what is my mistake?
Closure under isomorphisms means something different: if $A \prec B$ and $f : B \to B'$ is an isomorphism than $B', f(A) \in \mathcal K$ and $f(A) \prec B'$. You can check that this holds for every elementary class.