The objective of this work was to built a model for describing the transverse section of Norway spruce (Picea abies Karst.) stems, Le. the ring shapes and their variations within the trees. A set of 453 disks was sampled and more than 9500 ring's shapes were measured. Four différent elliptical models were analysed, and as a first result it was demonstrated that a single elliptical model was well adapted to fit the data. On this base, the ring ellipticity variations were analysed from the pith to the bark and from the bottom to the top of the tree. The rings are rather circular near the pith and at the top of the tree stem and rather elliptical close to the bark and at the bottom of the stem. As a second result we have built models describing these variations. These models could be further used on âne by laser sensors, for a better sawing pattern optimisation.