Melt blowing is a one step process which produces nonwoven fabrics by extruding polymer melt through a die with orifices and extending the extruded polymer melt with a jet of hot air that rapidly attenuates the extrudate into small diameter fibers. Polypropylene (PP) with different MFR were used in this research, and the melt blowing process was analyzed through the observation of spin-line near the spinneret by using a high speed camera. Periodic fluctuation of fiber diameter was observed under the conditions of preparing fine diameter fibers. For the PP of low and high viscosity, the factors dominating the instability of the process were found to be throughput rate and air flow rate, respectively. These results suggested the two different origins of instability, i.e. surface tension and melt draw ratio. For the low viscosity PP, the shot formation appeared to be suppressed with the reduction of nozzle diameter. This finding was confirmed not only by monitoring the spin-line using a high speed camera but also by observing the morphology of the fiber web using a scanning electron microscope. In other words, reduction of nozzle diameter contributed for the stabilization of the spinning process and improved uniformity of the diameter of fibers in the web. Oppositely, for the high viscosity PP, the spinning process tended to be unstable with the reduction of nozzle diameter. This result indicated that the stability of the high viscosity PP cannot be discussed only from the view point of melt draw ratio.