Traditional spark plug igniters are widely used in engine applications. This technology is better known among others, and some limitations appear in its control in terms of space and location of the deposited energy. An alternative laser igniter device to overcome this problem is more and more studied. The principle of this technology is to focalize a Gaussian laser beam in the center of a combustion chamber. However, optics used to focalize the laser beam are a crucial point to achieve ignition. The presented experimental work highlights the role of the beam diameter for the same laser source in the ignition process of a reactive mixture. In this paper, five beam diameters have been investigated for five equivalent ratios of n-decane/N-2/O-2 mixture at 1 bar and 347 K with the Langlie method. Results yield that ignition needs less and less energy sent from the laser source when the beam diameter is more and more expanded.