In this paper structural intensity technique is used to formulate a theoretical power flow model in a thin homogeneous naturally orthotropic plates for quasi-longitudinal waves. This method is proposed as an alternative to statistical energy analysis (SEA) or finite element analysis (FEA). The stuctural intensity technique is a tool for localization and characterization of sources and sinks as well as for identification and ranking of the transmission paths of vibration power flow. In practical applications plates are often reinforced by ribs or stiffeners or even corrugated to increase rigidity. These plates are therefore basically orthotropic plates with different elastic constants in the two orthogonal directions. Analysis of vibration power flow in orthotropic plate is necessary for the consideration of structure-borne vibration and noise propagation.