A holographic transmission grating is made using a 15‐mW He–Ne laser (Spectra Physics Model No. 124B). Two front‐surface mirrors are used to obtain two coherent beams by the method of division of wavefront. The interference pattern is recorded on a high‐resolution photographic plate. The value of ‘‘d’’ the grating constant is calculated using the relationship between the fringe spacing and the angle between the two interfering beams. This value is then compared with the one obtained by using the grating equation, a spectrometer, and a mercury lamp. From the results obtained and in view of the fact that He–Ne lasers are readily available in physics laboratories, this experiment is recommended as suitable for an advanced undergraduate optics and spectroscopy or quantum physics laboratory.