Commonly used method of unwoven fiber filter production is melt-blow technology. This method is based on extruding melted polymer into the stream of hot air where it forms fibers of diameter in the range of 1 to 50 micrometers. Properties of melted polymer strongly depend on its temperature, time of being melted, and how melted polymer has been extruded. Because of that, the diameter of the fibers extruded from the head can vary in time and between each nozzle. During the investigation of ready filter it is difficult and sometimes too late to estimate the fiber diameter. The only way to keep parameters in time and to investigate the process and then to improve it, is to measure fibers diameter on line, during the process of filter production.The principle of fiber diameter measurement is based on a scattering of laser light. Theoretical dependence of light intensity as a function of scattering angle can be found by solving Maxwell equations. Equations are solved separately for both polarization vectors by developing magnetic and electric field expressions into Fourier-Bessel series (Bohren and Hauffman, 1983). Parallel and perpendicular polarization components of scattered light are as follows:Light is scattered only in the plane perpendicular to the fiber axis. Normalized scattered light intensity as a function of the scattering angle, for some diameters of the fibers can be seen on a draw.Fibers stream under a head of melt blown stand is irradiated by a laser diode in short pulses to avoid coincidence. During these pulses data from linear matrix of photodiodes are collected. From this data a fiber diameter can be calculated. The point of measurement can be easily moved along a row of nozzles. On-line measurement of fiber diameter can improve quality and repeatability of filters obtained in melt-blow stand. This methods also lets to investigate the process of fibers spinning to obtain better aerosol filters.