A mathematical model that includes crystallization in the spinline and the effect of crystallization on the extensional viscosity and the various physical properties of polypropylene has been developed and used to help in identifying the various factors that can affect the spun yarn characteristics. The model is used to simulate effects of spinning parameters on fiber physical properties, temperature, and stresses. The experimental observation of a minimum in density of the spun yam at high throughput rates, when density is plotted as a function of takeup velocity, has been investigated in some detail. It has been found that all conditions which can substantially affect the rate of cooling and the orientation of the polymer in the spinline, viz, throughput rate, spinning temperature, and spinning speed have an important bearing on the temperature range in which crystallization can take place in the spinline and thus affect the density. It is suggested that in addition to these factors, the formation of different crystal modifications at different spinning speeds could also contribute to the reduction in density of these samples. The model cannot reflect the observation of density changes occurring due to the formation of different crystal modifications. Nevertheless, it can be of use in understanding the effects of various process conditions on the cooling rate and the orientation of the polymer in the spinline.