Terahertz technologies are believed to find various applications such as security screening, medical imaging, nondestructive inspection etc. For high speed, high sensitivity, and portable terahertz imaging applications, Terahertz Focal Plane Arrays(THz-FPAs) were designed and demonstrated based on self-mixing mechanism in AlGaN/GaN high-electron-mobility transistors (HEMT). The THz-FPAs with an array size of 32×32 were realized by flip-chip bonding a detector array and a readout integrated circuit(ROIC) based on silicon CMOS technology. Each pixel detector consisted of two HEMTs which were configured in a differential output scheme so as to enhance the voltage responsivity, reject the common-mode voltage noise, and increase the sensitivity. The differential detector signals were amplified in the ROIC and converted to digital data by an Analog-to-Digital Converter(ADC) in a printed-circuit board(PCB) which also hosted a Field-Programmable Gate Array (FPGA) for data acquisition and conversion. The data converted into a video stream was sent to a personal computer through the Camera Link interface. The focal plane array was used to demonstrate imaging of terahertz beam spot, terahertz interference ring pattern, and a rotating plastic blade under terahertz illumination all with a frame rate up to 30 Hz.