The effect of temperature fluctuation in silicon melt on the electromotive force (E) of oxygen sensors was examined by experimental and analytical studies. Two different reference materials, namely, Cr/Cr2O3 and Ni/NiO, were used to clarify how the temperature fluctuation in the melt affects the electromotive force of the sensors. The temperature derivative of the electromotive force (partial derivative E/partial derivative T) of the reference material Cr/Cr2O3 is larger than that of Ni/NiO. The limits of sensors using Cr/Cr2O3 or Ni/NiO as a reference material are discussed from the standpoint of the temperature fluctuation in the melt. The degradation mechanism of the sensors is also discussed. An apparent oxygen partial pressure gradually increased with elapsed time of 40 min from the start of measurement due to the degradation of the sensors. We investigated the cause of the increase of partial pressure based on the metallization of reference materials in the sensors.