We propose a Dynamically Biased Multi Threshold CMOS (DBMT) technique for power gating in FDSOI. In DBMT, effective threshold voltage of a high-Vt power switch transistor is lowered by forward body biasing (FBB) to improve performance at the operation, while it is raised by reverse body biasing (RBB) to further reduce leakage in the sleep state. We applied this technique to a 32-bit multiplier circuit of an experimental CPU Geyser-1 in which the multiplier is power gated at run time in a fine-grained manner during the CPU operation . Simulated results in 65nm FDSOI with thin BOX revealed that the area of the power switch (PS) in DBMT technique can be reduced by 55% as compared to the conventional MTCMOS when suppressing the delay increase due to PS insertion within 10%. DBMT reduces leakage energy of the multiplier considering the energy overhead by up to 55% as compared to the MTCMOS when running application programs.