The light of laser source is not completely single frequency and has limited linewidth. Lasers linewidth might be up to several hundreds of GHz. Today, with the development of laser systems, construction and use of narrow linewidth lasers are rapidly developing. Therefore introduction of methods of precise measurements of frequency spectrum and noise characteristics of these lasers, seems to be an important need. Recently, lasers with linewidth less than 100 kHz are commercialized. One of the challenges that face manufacturers is accurate linewidth measurement of such lasers. Optical spectrum analyzer has the accuracy of a few tens of megahertz. Therefore (for narrow linewidth laser), another measurement methods based on electrical devices (electrical spectrum analyzer ESA) are replaced. To measure the linewidth of the narrow linewidth lasers, due to limited bandwidth of ESA, measurement methods based on interference are employed. The main idea of this method is based on the conversion of phase or frequency fluctuations into intensity changes in an interferometer such as Mach-Zehnder or Michelson. In general, the source of laser frequency noise can be considered, white noise (Lorentzian) and 1/f noise (Gaussian). Many applications such as coherent communications systems is limited by the white component of the frequency noise. Because the phase or frequency fluctuation resulting from 1/f noise are very small and by using the frequency tracking circuits and PLL circuits could be compensated. Therefore, measuring the spectrum broadening caused by white noise (Lorentzian linewidth) as a result of random changes in phase laser beam is in important to design data transfer systems. In this report, various methods of the linewidth measuring of the narrow linewidth laser, have been studied. Then according to the existing facilities, a modified delayed self homodyne interferometer DSHOI, using a phase modulator and delay fiber length less than the coherence length of the laser in a Mach-Zehnder interfrometer, in order to calculate the Lorentzian linewidth, is designed and implemented. In the interferometer system with a short delay fiber, low frequency noise (1/f) of laser are filtered from beat signal then measurement of spectrum broadening caused by white noise and Lorentzian linewidth along with non-linear fitting process provided. If the dominant noise of laser is 1/f, such as high power laser and fiber laser, this method is not suitable for linewidth measurement. Measurement for the narrowest accessible laser in the laboratory (linewidth of semiconductor laser is 85 kHz), was performed. Error of measurement with designed system is less than 2%. The study showed that by increasing the amount of delay in the interferometer, measurement error due to nonlinear effects of polarization and instability in the system increases. Resolution of designed measurement system is approximately 30 kHz. Keywords: Narrow linewidth laser, lorantzian lineshape, DSHOI, DSHI