Tribological properties of diamond coatings are very complex; if the wear rates remain very low, the friction coefficient is in a large range of values. This behaviour can be explained by: (a) the deposition method: the way the dangling bonds are saturated is not the same depending on whether diamond is obtained by chemical vapour deposition (hydrogen saturation) or combustion flame method (oxygen saturation); (b) by the intrinsic properties of diamond crystals which also take a predominant part in the tribological behaviour. In particular, a change in the growth direction induces different physico-chemical properties for the coatings, and consequently modifies their friction coefficient and its evolution with time. The aim of this paper is to highlight the influence of two particular crystal orientations, {111} and {100}, on the friction of the diamond-coated pin/high-speed steel disc couple and on the existence and the importance of the transferred layers. The role of two of the experimental parameters was also studied by realising the tribological tests under various environments (vacuum, oxygen and water vapour) and by applying different normal loads (0.2 N and 1.4 N): the water vapour exposition under low load induces different results for the {111} and {100} oriented coatings. The sliding surfaces were also observed by SEM and analysed by EDS to study the sight and the composition of the transferred layers.