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We have characterized performance of on-chip circular silicon transformers. The circular transformers were fabricated by the standard 0.18 mum CMOS process. We used measured two-port S-parameters, a de-embedding procedure, and simulated S-parameters derived from a lumped-element circuit model. We examined performance indicators of transformers such as quality factor (Q), maximum available gain (G max), coupling coefficient (kappa), and minimum noise figure (NF min) for various cases. We characterized all the parasitic capacitive and inductive effects. The results obtained are useful in the global optimization of high-performance transformers used in the design of certain radio-frequency integrated circuits.