An experimental thermal characterization method is developed for high thermal conductivity thin films. The method utilizes Ta/Pt resistors on microfabricated free-standing thin film structures both for heating and temperature monitoring at different positions on the structures. The steady-state temperature at the heater and the sensor positions are monitored as a function of the power dissipated by the heater under vacuum environment, and the thermal conductivity is estimated by comparing these results to FEA and/or analytical models. The developed method is used to characterize the thermal conductivity of various different CVD diamond films of different grain sizes and films thicknesses. The measured thermal conductivity values range from 15 W/m·K to 300 W/m·K, which are at least one order of magnitude lower than that of natural diamond. It is also shown that the thermal conductivity of such films in the in-plane direction increases with increasing grain size and film.