The Direct Metal Laser Sintering process (DMLS) is a Rapid Manufacturing (RM) technique. A 3D object is built layer by layer through laser consolidation of metallic powders. Several industrial DMLS stations are commercialised. The current performances of the parts manufactured by this process remain insufficient. Hybrid processes were developed. The DMLS process is then coupled to other technologies in order to post-process DMLS parts. The quality of the parts is improved but the post-processing stage slows down the manufacture. The main goal of this Ph.D. work was to increase the understanding of this laser manufacturing process in order to be able to make DMLS parts that could be used without post-processing. The powder used in this work is a 316L grade Stainless Steel powder. Particles are spherical and have a diameter between 11 and 45 µm. The powder has a good flowability, leading to good powder layer quality after deposition (before laser consolidation). Many monolayer samples were built, using more than seventy sets of process parameters. For each set of parameter, a specific microstructure is obtained. A microstructural index η was defined. The index is an adimensional number between 0 and 1 and represents the degree of sintering (η is close to 0 if the sintering process just began, η tends towards 1 if the sample's density is close to full density). This index allows us to characterize quantitatively the microstructure of all the parts we built. Three key parameters were identified : the pulse duration τP, the energy density of the laser Eρ and the powder bed density ρlit. Thanks to the relation η = η(τP, Eρ, ρlit) established, we estimate quantitatively that Eρ is the process parameter which has the largest influence. Qualitatively we developed phenomenological models predicting locally that the microstructure of a monolayer is becoming finer if the pulse duration as well as the powder bed density increase and if the energy density of the laser decreases.