The photoacoustic (PA) method is useful for measuring the thermal characteristics of ultra-thin films, because it utilizes critical damping of thermal waves. The thermal characterization of a diamond film deposited by the chemical vapor deposition (CVD) method was performed by using this method. We obtained the thermal image and measured the thermal conductivity in the vertical-depth direction of a diamond film deposited on silicon. The thermal conductivities for two samples (2 and 50 mu m) were obtained as 396 and 639 W m(-1) K-1, respectively. In addition, we measured the phase differences of the PA signal between Pt implanted diamond film and silicon, and non-implanted diamond film and silicon. P+ implanted film produced increases of the phase difference. These increases were nor affected by a heat treatment at 1000 degrees C for 10 min.