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We investigate the two methods of defining the surface gravity κ on the horizon, which are the metric definition and the definition given by the spacetime conformal method. It is found that the latter is of greater generality. By this method, we find that κ ofη- ξ spacetime is equal to the exponent factor α in the coordinate transformation,which confirms the argument that η- ξ spacetime can be considered as the background spacetime for finitetemperature field theories. The reasons why the metric definition of κ can not be applied in η - ξ spacetime are presented.