An adaptive channel access control method for CDMA/PRMA protocol is proposed. The proposed method utilizes a load and backlog based access control. Dynamic optimal channel loads by which the required packet loss probability can be satisfied are obtained. The number of contending terminals is also estimated more accurately, using statistical characteristics of source models. Permission probability is then calculated based on the dynamic optimal channel load and the estimated number of contending terminals such that the mean channel load (the mean number of packets transmitted in each time slot) can be maintained at the optimal channel load. By maintaining the mean channel load at the dynamic optimal channel load, the radio channel can be very effectively utilized, satisfying the required packet loss probability. A backlog based data transmission using a mixed mode of contention and reservation mode is also proposed to reduce redundant contention and corruption. Simulations are carried out in an isolated cell environment and a cellular environment. The simulation results show that the system capacity can be improved significantly by the proposed method compared with the conventional permission control methods.