Summary: In this paper, we analyze the throughput and delay performances of the CDMA unslotted ALOHA system considering packet retransmisson. We also clarify the stability of the system. Based on these results, we propose the optimal retransmission control (ORC) to improve the performances. The ORC is the scheme to prevent the system from drifting to an undesirable operating point by controlling the birth rate of retransmitted packets. As a result, it is shown that the throughput and delay performances of the system with the ORC are better than without the ORC and the system does not drift to an undesirable operating point.