This paper describes a programme of mechanical tests undertaken to assess the relationship between mechanical properties and physical characteristics in the longitudinal direction of fast-grown plantation spruce grown in Ireland. Little published work exists in the literature in relation to characterisation of such fast-grown material. The parameters studied include density, knot area ratio, modulus of elasticity and ultimate strength. Modulus of elasticity is the most highly correlated parameter to the tensile strength for both clear and in-grade specimens. The knot area ratio has a considerable influence on the strength of the timber both in compression and tension. Density is more highly correlated to ultimate compressive strength than ultimate tensile strength in clear wood specimens. The modulus of elasticity in tension has a poor correlation to the density of clear wood and is also poorly correlated to the knot area ratio and the density of in-grade specimens. The information produced is essential in the future development, design and optimisation of engineered wood products using such fast-grown plantation wood.