Paleomagnetic analyses have suggested that the lunar magnetic field underwent a significantchange from 4.25 to 3.19 Ga, indicating the rapid transition of the lunar dynamo mechanism. We used the vander Pauw (vdP) method to measure the electrical resistivity of Fe‐S‐P alloys under conditions relevant to thelunar core and estimated the thermal conductivity of the Fe‐S‐P lunar core. These values were incorporated intothermal and dynamo models to investigate the evolution of the lunar core. Our model indicates that the innercore began to grow as early as 4.35 Ga, the solidification regime switched at 3.50 Ga, and the thermal dynamoceased between 3.78 and 3.51 Ga. The cessation of the dynamo could be due to a low buoyancy flux andinsufficient entropy dissipation. Thermal and compositional dynamos cannot sustain the ancient strength of theMoon's magnetic field, and require other energy sources.