A single-slit diffraction experiment in an introductory physics laboratory is generally comprised of a rectangular slit and a laser source. The laser beam is sent to the slit producing the well-known diffraction pattern on the screen. This paper proposes a simple modification of the single-slit diffraction experiment to increase student attention about the laser beam and its diffraction. A single slit can be used to measure laser beam width. The beam width measurement is demonstrated by using a 632.8 nm Helium Neon (HeNe) laser. In this experiment, the laser beam is assumed as a Gaussian beam. The beam width is determined by scanning in one direction across the laser beam. The measurement results are shown to achieve satisfactory results when compared with a commercial laser beam profiler. By measuring the laser beam widths, students will qualitatively observe that the laser beam width increases along propagation distance. This experiment can be an additional exercise in a single-slit diffraction experiment.