For mobile devices, this paper proposes a compact hardware (H/W) implementation for the KASUMI block cipher, which is the 3GPP standard encryption algorithm. In [4], Yamamoto et al. proposed the method of reducing temporary registers for the MISTY1 FO function (YYI-08), and implemented a very compact MISTY1 H/W. This paper aims to design the smallest KASUMI H/W by the application of YYI-08 to KASUMI, which has a similarly structured MISTY1 FO function. We discussed the applicability and found the problems on register competition and logical equivalence in the simple application, so we propose the new YYI-08 improved for KASUMI and the compact H/W architecture. According to our logic synthesis on a 0.11-μm ASIC process, the gate size is 2.99 Kgates, which is the smallest as far as we know.