In this work, a simple optical method for measuring particle size distribution is investigated, as part of a particulate matter monitor. Particulate matter, defined as the suspended solid or liquid particles in air, is widely considered as one of the main air pollutants and a source of smog and global warming. Different particle sizes pose different health hazards, where particles with particle diameters less than 10 µm (PM10) can enter the nasal cavity, those smaller than 7 µm (PM7) can penetrate the throat and those smaller than 2.5 µm (PM2.5) can enter the lungs. These health risks have driven global interest in monitoring PM sizes and concentrations. The setup used in this work consists of a laser source in the visible spectral regime (633 nm wavelength) shined on opaque particulate matter of diameter distribution in the order of 10 µm to 50 µm. The diffraction pattern in the far field is captured using a CCD camera, without stringent requirements on the pixel size of the camera or large number of pixels. The captured image is processed to identify the diffraction pattern due to each particular matter, from which the size can be retrieved. This method offers an inexpensive, non-intrusive and simple implementation to finding the size distribution of particulate matter. Having good potential for further improvement and system miniaturization, particle sizing using laser diffraction pattern analysis opens the door for cheap personal air quality monitoring for real-time consumer application.