A polymerase chain reaction (PCR) chamber microfabricated into a silicon substrate is presented. The unique design features of the device provide excellent thermal isolation of the PCR chamber from the surrounding silicon substrate allowing for highly localized control of the PCR chamber temperature. Steady-state, thermal, finite element method simulations were conducted to optimize the microreactor design. Silicon microreactors were fabricated and experimentally characterized. The fabricated PCR reactor is demonstrated to have good thermal isolation between the microcavity and surrounding silicon chip. Successful amplification of human genomic DNA is also demonstrated.