The girth plays an important role in the design of LDPC codes. In order to determine the girth of Tanner(5,7) quasi-cyclic( QC) LDPC codes with length 7p for p being a prime with the form 35 m + 1,the cycles of lengths 4,6,8,and 10 are analyzed. Then these cycles are classified into sixteen categories,each of which can be expressed as an ordered block sequence,or a certain type. It is also shown that the existence of these cycles is equal to polynomial equations over Fpwho has a 35th unit root. We check if these polynomial equations have a 35th unit root and obtain the girth values of Tanner(5,7) QC LDPC codes. The girth plays an important role in the design of LDPC codes. In order to determine the girth of Tanner (5,7) quasi-cyclic (QC) LDPC codes with length 7p for p being a prime with the form 35 m + 1, the cycles of lengths 4, 6, 8, and 10 are analyzed. Then these cycles are classified into sixteen categories, each of which can be expressed as an ordered block sequence, or a certain type. It is also shown that the existence of these cycles is equal to polynomial equations over Fpwho has a 35th unit root. We check if these polynomial equations have a 35th unit root and obtain the girth values ​​of Tanner (5,7) QC LDPC codes.