The magnetic alignment of poly(ethylene terephthalate) in molten state was studied by in-situ birefringence measurement in a magnetic field. The melting point of the sample determined by DSC was 253°C. We observed the increase of birefringence in a molten state at temperatures between 254°Cand 260°C in the magnetic filed (10T). The dependence of the apparent rate of orientation on the annealing time was attributed to the increase in the viscosity due to the structure formation under the lower supercooling condition. The time at which the transmitting light became a maximum increased with the increase in the melting temperature. These results suggested that the residual structure of crystal, which can be aligned magnetically, existed in the molten state and the volume fraction of the residual structure was reduced with increase in the melting temperature.