This paper investigates the construction and iterative threshold decoding of low rate Quasi-Cyclic One Step Majority logic codes based on combinatorial designs. These codes are constructed with two type of difference family: cyclic disjoint difference sets and cyclic disjoint difference family. They can be encoded with low complexity, and perform very well when decoded with the Iterative threshold decoding (ITD) algorithm. We present an amelioration of ITD algorithm for decoding these codes. This algorithm modification can be applicable for every code rate. The performances of this algorithm for decoding these codes on Additive White Gaussian Noise (AWGN) channel, is investigated.