The conductivities of three PEO-NaSCN samples whose molecular weight are 6000, 12000 and 20000 respectively have been studied as a function of temperature during two thermal cycles. It was found that the completeness of complex reaction in the samples had a obvious effect on the electrical behaviours. The complex reaction in molecular weight 12000 sample was complete, therefore the electrical property was not influenced by thermal history. While this reaction in the other two samples did not fully complete and can occure in certain temperature range during the first heating process, thus rather complicated log σ versus 1/T curves were observed. It was noted that the extra PEO content in the samples had a remarkable influence on the electrical property. Molecular weight 12000 sample contains more extra PEO content than the others. Therefore it has the highest conductivity and the logσ versus 1/T curve consists of two straight lines with a discontinuity at temperature near the melting point of pure PEO. While the extra PEO content in molecular weight 6000 sample is much less, it is responsible for the lowest conductivity and continual change of conductivity with temperature.