Acoustic emission(AE) behavior during the tensile deformation of Al-Li 8090 alloy was investigated to clarify the acoustic emission source, and to correlate the acoustic emission activities with the deformation mechanisms. Continuous AE near yield point showed maximum at the initial aging stage and decreased with further aging. The decrease of continuous AE activity was related with the change of deformation mechanism from planar slip to by-pass. Continuous AE was considered to occur because the shearable δ` precipitates were acting as breakable pins for dislocation movement. On the other hand, burst AE coincident with the load-drop of serration during the post-yield deformation in under-aged condition was observed, and the source was thought to be the deformation localization crossing few grain boundaries through the slip plane work-softened by shearing of δ` precipitates near yield paint.