A variety of mobile data communication services based on cellular phones and the PHS (Personal Handyphone System) have recently been developed and used widely. The maximum transmission rate in public mobile data communication services is currently limited to 64 kbit/s, but higher transmission rate will be needed in order to meet the requirements of mobile multimedia applications. We have therefore developed 384 kbit/s-PHS experimental system that uses the 64 kbit/s PHS data communication protocol (PIAFS) and the PPP Multilink protocol. This paper presents the implementation and performance evaluation of the 384 kbit/s-PHS experimental system. Throughtput performance of the system is evaluated using FTP under various radio propagation environments.