Low-density parity-check (LDPC) codes are considerable since these codes achieve near-capacity performance with iterative message - passing decoding algorithms and there are methods to produce codes with block lenght of adequate sizes . Among the methods for constructing LDPC codes , randomly constructed ones have weaknesses as far as storing and accessing a large parity-check matrix , encoding data and analyzing code performance is concerned . Designing these codes via algebraic structures solves some of the aforementioned problems . In recent years , several algebraic methods have been presented for constructing LDPC codes . Among them , the structure of [155 , 64 , 20]-code , which is a quasi-cyclic (QC) LDPC code designed by Tanner in refrence [2] , is considered in this thesis . The parity-check matrix of this code contains some blocks of circulant matrices , so that it naturally induces quasi-cyclic property into the code and designs a suitable encoder . In this dissertation , based upon the structure of the parity-check matrix of the [155 , 64 , 20]-code , a generalization of the method is provided in order to obtain a large continuous encoding and decoding are among the important capabilities of these codes . In the third chapter of this thesis , based on the algebraic structure of the parity-check matrix of regular QC LDPC codes and the related convolutional codes , some bounds on the girth and minimum distance of these codes are determined . In the last chapter, by checking the performance of the constructed codes , it is shown that the algebraically constructed QC LDPC codes compared to random regular LDPC codes at small to average block lengths have superior performance. Also the performance of the LDPC convolutional codes determined based on continuous decoding procedure is better than that of the basic QC LDPC codes. In the end , it is shown that via increasing the size of each circulant block in the parity-check matrix of the basic QC block code to in?nity , the LDPC convolutional code can be considered as the unwrapped form of the basic QC LDPC code.