Theoretical analysis of the fibre motion in the draft zone has been carried out and the basic equation expressing the number of fibres over the cross section in the draft zone has been derived under the assumption that the sliver is a continuous medium. The denisty function of speed change fibres included in the average velocity term has been assumed in four kinds, i. e. uniform, triangle, cosine and normal. The theoretical values of the number of fibres over the cross section were compared with the experimental. The following conclusions were obtained: 1. The theoretical equation had good approximation to the experiment. 2. The fibre number distribution over the cross section the draft zone was not affected by the types of four density functions of speed change fibres.