Drift tube is the most important part in ion mobility spectrometer. It is usually composed of several successive conductive and insulator rings. The adjacent conductive rings are connected to each other by a resistor. A potential difference is applied between the two ends of the drift tube to create a homogeneous electric field inside the tube. Ions, produced by ionization source, pass through the drift tube to reach collector. Most ions are annihilated during their drift towards the collector. Any improvement on transmission of ions through drift tube results in considerable enhancement in ion signal intensity. The ion lost and its mechanism is not well understood. The aim of this work is to study and characterize the ion lost due to length of drift tube and the electric field. The results show that increasing the drift field enhances the collector current quadraticly. It was also shown that the ion current decreases with the drift tube length. A simple model based on kinetics of two parallel processes (loss or transmission) is proposed to explain the results. In this model it is assumed that the ion lost rate is proportional to the ion density and the ion transmission rate is proportional to the ion velocity as well as the ion density. The collector current I C is then described by; Where I o is the initial current delivered at the entrance of the drift tube, u is the velocity of ion and k d and k L are the ion lost and ion transmission rate constant, respectively. k d and k L are constants which depends solely on the geometry of drift tube. All the observed results are in good agreement with the predicted behavior by the model. The ratio k L / k d which is a measure of ion lost, was assumed to be dependent on d , the drift tube length. Then the above equation was rearranged to Where g is a new constant depending on geometry of drift tube. This equation predicts that 1/I o is linearly proportional to the drift length, with an intercept of 1/I o . The observed results well agree with this prediction. In addition, it was proved that I o is linearly proportional to the applied electric field strength. It is thought that ions are mainly lost due to collision with wall. This is due to transversal expansion of ions moving through the drift tube. An experiment was performed to measure the radial distribution of ions at different distances from the ion source. The result which is a 3-D image of the ion distribution in the drift tube, relates that the ions become broader as the ion cloud travels. As a result they hit the walls of the drift tube and are annihilated. The results obtained are used in designing drift tubes for special applications.