In this paper, we study the cycle structure of (3, 19)-regular Tanner's quasi-cyclic (QC) LDPC codes with code length 19p, where p is a prime and p ≡ 1 (mod 57), and transform the conditions for the existence of cycles of lengths not more than 10 into polynomial equations in a 57th root of unity of the prime field F p. By employing the Euclidean division algorithm to check whether these equations have solutions over the prime field F p , the girth values of (3, 19)-regular Tanner's QC-LDPC codes of code length 19p are determined. In order to show the good performance of this class of QC-LDPC codes, numerical results are also provided. INDEX TERMS QC-LDPC codes, Tanner graph, girth, prime field I. INTRODUCTION Quasi-cyclic (QC) LDPC codes [1] are a class of wellknown channel codes, and widely used in the communication and storage systems because of their excellent features: lowcomplexity encoding and decoding algorithms [2]-[5], good performance in the waterfall and error-floor regions [6]-[10], and easy implementations in hardware [11]-[13]. In general, the low-complexity algorithms for decoding LDPC codes are under the frame of iterative decoding. However, short cycles in the Tanner graph [14] of an LDPC code degrade the iterative decoding performance. Hence, constructing large girth LDPC codes and/or determining their girths is of interest in coding theory and graph theory [15], [16].