It is attractive to produce a fat and hot plasma in a reversed-field configuration (RFC) by a small magnetic field since adiabatic compression – one of the best supplementary heating methods in RFC – is very effective. In the large-diameter theta pinch, a fat and hot (600–200 eV) RFC plasma could be produced in a small confining field (0.3 T). The heating mechanism was examined. It was found that the delayed pinch effect and the Joule heating due to anomalous plasma resistivity enable the production of such a plasma.