Melt blowing is a one step process which produces nonwoven fabrics by extending the extruded polymer melt with a jet of hot air that rapidly attenuates the extrudate into small diameter fibers. Unstable behavior plays a important role to decide fiber diameter and its distribution on the melt blowing process. Thus unstable behavior of the process was investigated in this study. Stability of the process was evaluated by measurement of fiber diameter distribution in the resultant web. In addition, unstable thinning behavior of spin-line near the spinneret was directly observed by using a high-speed camera. Broadening of fiber diameter distribution was observed in the spinning conditions of small throughput rate or high airflow rate. Periodic fluctuation of fiber diameter was also observed in those spinning conditions. It indicated that the diameter distribution was caused by instability of spin-line. For the PP of low viscosity, instability of the process dominated by throughput rate. On the other hand, airflow rate was more effective on instability of high viscosity PP. These results suggested that the two different origins were existed. Results of numerical analysis suggested that the origin of instability of small throughput rate with low viscosity PP was surface tension of molten polymer. In addition, the other origin of instability noticeable in high airflow rate was resonance with turbulence of blowing air.