The authors developed an experimental method that can measure the in-plane thermal conductivity of a thin film and the interface thermal conductance between the film and the metal strip, simultaneously. This technique, the in-plane 3 omega method, can be applied to the films with very high thermal conductivity such as diamond films. To guarantee the reliability of the measurement, the factors causing error were analyzed rigorously. Then, the method was demonstrated on silicon dioxide and silicon nitride films and was valid in experiments performed in the open atmosphere. They also applied the method on several chemical vapor deposited diamond films of different thicknesses and pretreatment methods. The data are comparable with those from previous researches.