With a lot of millimeter-wave (mm-Wave) applications being issued, wideband circuits and systems have attracted much attention because of their strong applicability and versatility. In this paper, four transformer-based ultra-wideband mm-Wave circuits demonstrated in CMOS technologies are reviewed from theoretical analysis, implementation, to performance. First, we introduce a mm-Wave low-noise amplifier with transformer-based Gm-boosting and pole-tuning techniques. It achieves wide operating bandwidth, low noise figure, and good gain performance. Second, we review an injection-current-boosting tech-nique which can significantly increase the locking range of mm-Wave injection-locked frequency triplers. Based on the injection- locked principle, we also discuss an ultra-wideband mm-Wave divider with the transformer-based high-order resonator. Finally, an E-band up-conversion mixer is presented; using the two-path transconductance stage and transformer-based load, it obtains good linearity and a large operating band.