The purpose of the present work is to study the relation between the microstructure and the martensitic transformation in a binary NiTi alloy. Moreover, it has been conducted on wires which diameters are between 1.52mm and 50µm in order to demonstrate the possible appearance of a size effect. The transformation temperatures during the martensitic transformation in wires made of Ti-51.14at.% Ni have been studied as a function of the wire's diameter, the temperature and the duration of the heat treatment. The as-received wires are cold drawn but the amount of residual stress can vary from one to the other between 10 and 40%. Results obtained using differential thermal analysis (DTA) have not shown any size effect on the martensitic transformation. One does, however, observe small changes that can be attributed to the residual stresses after cold-drawing. There is a strong effect of the temperature associated with the heat treatment. When the annealing is performed below the recrystallisation temperature (550°C), the transformation proceeds through a complete R-phase transformation followed by the appearance of the martensite, and the transformation temperatures increase as a fonction of the heat treatment temperature. If it is above this temperature only a partial R-phase transformation occurs together with a slight decrease or even a stabilization of the transformation temperatures. When the ageing time, performed at 520°C, is prolonged (up to 40 hours) the transformation temperatures increase and incoherent precipitation of Ni4Ti3 and Ni3Ti2 is observed. To avoid residual stresses, samples have been solution treated at 900°C followed by water quenching and further aged between 350°C and 550°C. DTA measurements have shown that the characteristics, as well as, the transformation sequence are strongly affected by the annealing temperature during this second treatment. The transformation temperatures increase together with the appearance of a multiple step transformation. This new behaviour of the transformation is linked to a modification of the microstructure of the samples. Transmission electron microscopy (TEM) observations have revealed after the aging treatment the occurrence of fine coherent precipitates of the Ni4Ti3 type. In-situ TEM observations on a sample showing the multiple step transformation have shown that both the R-phase and the martensitic transformations are strongly affected by the presence of these precipitates. The first effect of the precipitation process is the depletion in the nickel content of the matrix which is accompanied by an increase of the transformation temperatures. The second effect is the occurrence of a multiple step transformation which is due to the presence of a local stress field around precipitates which modify the local thermodynamic equilibrium and promotes the transformation. The R-phase to martensite transition then proceeds locally at a higher temperature, leading to the appearance of the three step transformation. This work has demonstrated the importance of the microstructure on the thermodynamic equilibrium and its effect on the transformation sequence. It also provides evidence for a transformation model based on the presence of internal stresses surrounding precipitates of the Ni4Ti3 type.