Let $A=kQ/I$ be a string algebra. We show that, if for any vertex $v$ of its bound quiver $(Q,I)$, there exists at most one arrow (resp. at most two arrows) ending with $v$ and there exist at most two arrows (resp. at most one arrow) starting with $v$, then the number of indecomposable modules over $A$ is ${\dim }_{k}A+\mathrm{\Sigma }$, where $\mathrm{\Sigma }$ is induced by $radP(v)$ (resp. $E(v)/\mathrm{s}\mathrm{o}\mathrm{c}E(v)$) with decomposable socle (resp. top), where $P(v)$ (resp. $E(v)$) is the indecomposable projective (resp. injective) module corresponded by the vertex $v$.