S. Kim et al. have been analyzed the girth of some algebraically structured quasi-cyclic (QC) low-density parity-check (LDPC) codes, i.e. Tanner $(3,5)$ of length $5p$, where $p$ is a prime of the form $15m+1$. In this paper, by extension this method to Tanner $(3,7)$ codes of length $7p$, where $p$ is a prime of the form $21m+ 1$, the girth values of Tanner $(3,7)$ codes will be derived. As an advantage, the rate of Tanner $(3,7)$ codes is about $0.17$ more than the rate of Tanner $(3,5)$ codes.