The average lower domination number $\gamma_{av}(G)$ is defined as \[ \frac1{|V(G)|}\Sigma_{v\in V(G)\gamma_v(G)} \] where $\gamma_v(G)$ is the minimum cardinality of a maximal dominating set that contains $v$. In this paper, the average lower domination number of complete $k$-ary tree and $B_n$ tree are calculated. Moreover we obtain the $\gamma_{av}(G^*)$ for thorn graph $G^*$. Finally we compute the $\gamma_{av}(G_1+G_2)$ of $G_1$ and $G_2$.