A small thermocyling system to perform DNA amplification by polymerase chain reaction (PCR) is presented in this report. PCR reactants are convected along a triangular closed-loop channel in a polymer chip by the thermosiphon effect. In an effort to develop a convection-based PCR for real application, we adopted a molded channel to define the flow path inside the chip, so that the chip may be suitable for disposability together with the merits of LOC; mass production, versatile integration and facile handling. We developed the geometry of the flow loop that made it easier to load the PCR reactants without air pockets inside. Based on systematic simulations and theoretical considerations of buoyant flows, the loop channel was designed to acquire an optimized flow for PCR. A PCR sample was dropped on a chip to fill the loop channel, and the chip was inserted into a slot of a heating block unit that was composed of three metal blocks with different temperatures. The temperature differences within the closed loop gave rise to buoyancy differences and the liquid reactant continuously circulated along the closed loop by the thermosiphon effect. Because there was no loss of time among the temperature shifts in the reaction steps, approximately 10 min were sufficient for the detectable amplification of 127 bp target gene from 10 pg microl(-1) of PCR fragments. In addition, it took 20 min for the mass amplification of 470 bp gene from PCR fragments or genomic DNA. The entire PCR system is compact enough even to be palmtop because it requires only a tiny temperature controller for a self-actuated thermosiphon flow. This thermocycling system would be a prototypical model for the field application of PCR.