Lately much interest is focused on SOI SiGe HBT in high-speed low-power BiCMOS applications. The frequency characteristics of the core device, SOI SiGe HBT in BiCMOS, determine its operating speed. So, this paper studies the frequency characteristics of SOI SiGe HBT on the basis of our proposed device structure, and analyzes the frequency characteristics in terms of the collector capacitance model we established. It is found that: 1) The characteristic frequency of SOI SiGe HBT increases with increasing doping concentration in the collector; 2) variations between the characteristic frequency and collector current of SOI SiGe HBT are consistent with those of the traditional SiGe HBT; 3) under the normal operating conditions, the maximum oscillation frequency of SOI SiGe HBT (a collector region with 3×1017 cm-3 dopant) is greater than 140 GHz, and the characteristic frequency is greater than 60 GHz. Compared with the traditional SiGe HBT, the maximum value of the characteristic frequency is increased by 18.84%. The conclusions above can provide important references to the design and research of SOI SiGe HBT and BiCMOS.