Composite I-shaped beams are currently used in aerospace, ocean and civil engineering applications. Increasing the vibration damping of composite structures is a concerned need in those applications. This research is an effort to investigate the damping performance of composite I-shaped beams with cocured viscoelastic damping layers using finite element method and modal testing technique. A hybrid finite element model for a damped composite I-shaped beam was developed. Modal strain energy method was used to estimate the linear viscoelastic modal loss factors of the composite beams. A numerical parametric investigation was conducted to study the effects of various parameters on the dynamic properties of composite beams under free-free end boundary condition. Selective design parameters included inserting location and thickness of damping layers. Natural frequency and modal loss factor were also extracted by experimental modal analysis. Static tests were performed to obtain the loss of static stiffness for inserting soft viscoelastic layers. Experimental and analytical results show the inserting location of cocured damping layers has significant effects on the damping and there exists a critical damping layer thickness for optimal damping with less significant modal frequency decrease. Static tests results demonstrate a little loss of static stiffness for embedding low module damping layers. A careful selection of damping layer location and thickness is needed to optimize the damping benefits desirable and the mechanics stiffness reduction that can be tolerated for intergral damping composite structures.