In clonal plantations of Eucalyptus trees, inter-tree variations are limited but due to the very fast growing. Wood properties in young trees exhibit a very important variability depending on the specimen location inside the log; most of these variations are "time" dependant: - primary juvenility is related to tree age at the time of the creation by the apical bud of the new log portion at height H; - secondary juvenility is related to cambium age at the time of generation of a new xylem ring located at a distance R from the corresponding pith of the log portion; - heart wood formation (duraminisation) is related to wood age after cambia cell division. In temperate species, the determination of each age is made simple by the existence of annual growth limits. In most tropical species their exact determination in impossible, but in some instances a correspondence between primary and secondary growth units can at least be established. However, in a case like that of Eucalyptus this easier approach was made impossible due to continuous growth, both primary and secondary; thus one could only rely on length measurements such as H and R. Results from experimental and industrial clonal plantations in Congo give relationships between H and tree age on one side, R and cambial age on the other. From measurements on selected felled logs, a first set of data is available, on one clone, for the modelling of physical and mechanical properties as related to parameters H and R. This can be done in connection with SIMQUA model of INRA Nancy to model macroscopic properties of log or timber. Further measurements on a larger scale re planned in order to implement the data base and to study genetic as well as the influence of site and forestry practice on the intra-tree variations of wood properties.