A mathematical model was built and used to show the motion of particles in a cascade rotary drum dryer. In a cascade rotary drum the flights pick up the particles at a number of points in the lower half of the drum while, in the upper half, the particles fall freely. A model is derived where the drag force exerted on the particles throughout the falling period is emphasised. The motion of the particles in a rotary drum is described by three actions: Cascade; Kiln and Bouncing. In this study a horizontal rotary drum was used where both the kiln and bouncing actions have minimal effect, therefore, the focus is on the cascade motion of the particles. A characteristic of the model is the falling number which is found to be dependent on the curtain properties. The model has demonstrated its ability to predict the effect of many important parameters such as drying medium velocity, drum rotation speed, particle size and feeding flow rate. It has been shown that increasing the drying medium velocity by 2.5 times results in an 85% decrease in the residence time. Also, the number of falling is shown to be limited and a function of the drum rotation speed, in this case 0.59 falling per second. An important feature of this model is the ability to predict the mean resident time, contact time interval and the resting time interval. The maximum error between the predicted and the measured data was <10%.