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A new experimental method, named the thickness difference method, is proposed for measuring the thermal conductivity of thick films. This method is developed based on a logarithmic relation regarding the difference of the total thermal resistances and the ratio of the film thicknesses between two similar systems. Such a logarithmic relation is supported by theoretical analysis and is analogous to that appearing in the steady heat diffusion problem between two concentric cylinders. Its accuracy is confirmed to be acceptable when the film thickness is large enough compared to the heating-wire width and small enough compared to the substrate width. The error analysis shows that the relative errors are under 1% for all film Biot numbers greater than 60. Finally, an associated experiment is also set up, and the measurement results confirm the accuracy of the proposed thickness difference method.