In this paper we will present a method to determine the material properties of a wooden bar with rectangular cross-section using guided waves in the measurement. We modelled the wood as an orthotropic material with 9 independent constants. We determined the dispersion curves theoretically in the three-dimensional case using a semi-analytical finite element method. In our laboratory we excited transversal and longitudinal waves in wooden bars of 2.5-4 meters length by piezoceramic transducers. We measured the displacement or the velocity of the surface of the bar by a laser-interferometer. The dispersion curves of the bar are determined from the measurement by the linear prediction method. We related the dispersion curves and the material properties. We found the material properties by parametric model fitting.