A variable partition duplex scheme on packet reservation multiple access protocol (VPD-PRMA) is analyzed in this paper. We assume a four-state speech model for a conversational pair and successfully obtain performance measures by approximate Markovian analysis. Analytical results show that they quite fit simulation results; and VPD-PRMA can get higher statistical multiplexing gain than fixed partition duplex (FPD)-PRMA, due to the trunking effect. We further investigate the effect of design parameters of permission probability and enlarged reservation duration on system performance by computer simulation. Simulation results shows that it exists appropriate values for these two design parameters so that the packet dropping probability can be minimized. The adjustment of permission probability can greatly improve the performance of uplink traffic with slight deterioration of the performance of downlink traffic; the provision of enlarged reservation duration scheme can enhance the system performance.