Abstract: A process and apparatus for measuring the effective diameter of the guided mode of a monomode optical fibre. The apparatus utilizes a lighting source such as a tungsten filament or a quartz-halogen lamp in conjunction with a monochromator for selecting light of wavelength .gamma. from the lamp, as well as a variable aperture lens. The imaging of the source is performed on the input face of the fibre by means of the lens with the image having a size greater than the diameter of the fibre core. The light from the source falls on the fibre in accordance with a cone having an apex halfangle .phi.. The lighting power P(.phi.) transmitted by the fibre as measured as a function of (.phi.) and the effective diameter 2w.sub.0 is determined in accordance with a disclosed formula.