In mobile communications, the quality of signal is deteriorated by fading. The spatial diversity technique is used to reduce the degradation of the signal by fading. There are several techniques to realize the spatial diversity gain, such as Single-Input Multiple-Output (SIMO), Space-Time Block Code (STBC), and so on. The SIMO system realizes the receive diversity. The error correcting code can be combined with spatial diversity gain. Low-Density Parity Check (LDPC) codes can achieve good performance approaching Shannon limit. In particular, an irregular LDPC code with optimum degree distribution achieves better performance than a regular LDPC code. The optimum degree distribution of the irregular LDPC code depends both on the channel and the system. In this paper, we derive the Eb/N0 thresholds of a regular LDPC codes and the irregular LDPC codes for SIMO systems with several diversity orders. The E b/N0 threshold is the smallest Eb/N0 that the decoder can achieve the error free. We also derive the optimum degree distributions of the irregular LDPC codes for SIMO systems with several diversity orders. We show that with low diversity order, the optimum degree distributions of the irregular LDPC code depends on the diversity order largely, while with high diversity order, the optimum degree distribution depends both on the diversity order and the combining scheme largely. We also show that comparing the optimum degree distributions for MRC and STBC, when the diversity orders are the same, the optimum degree distributions are also the same.