ABSTRACT The demands of today's 5G mobile network, especially low latency and high bandwidth, are a big challenge for the 5G Core (5GC) provider. The most critical user data packet handler in the 5GC network function (NF) is the user plane function (UPF), which is responsible for moving data from the user equipment to the destination data network, and vice versa. Existing work mainly focuses on implementing UPF using the key technologies of high‐speed data processing. In this paper, with a mobile core provider called free5GC for a stand‐alone (SA) 5G network, we share our experience with the implementation of UPF by using a programmable hardware appliance, which can offer more Tbps compared with the implementation of software UPF that can offer only a few hundred Gbps. For that, we demonstrate how to build up a more flexible architecture of UPF by using the software‐defined networking (SDN) concept due to the opacity of protocol specification. We split the UPF control signal implementation into a software application and user data packet processing into a programmable hardware appliance. We also show how to integrate a number of current UPF data plane free5GC implementations, such as Data Plane Development Kit (DPDK), Linux kernel module, and SmartNIC. Furthermore, we analyze and make use of microservices to support the specific features of the UPF data plane that cannot be implemented in a programmable hardware appliance. We tested our free5GC mobile network and the new UPF design architecture that can run on a real programmable hardware appliance from Accton CSP‐7551. The evaluation results show that our programmable user plane can reach the line rate.