In natural gas refining, dehydration is one of the most important processes. Many methods exist for dehydration, at the present using adsorption as an appropriate process of dehydration of natural gas is being improved. In this thesis, modeling and simulation of adsorption bed have been done to remove moisture from natural gas using 3A and 5A molecular sieves. This process is used in dehydration unit of South Pars Gas Complex, Phases 4 5. The gas is dried in two of the three dryers operating in parallel. At any time, two beds are in adsorption phase and one in regeneration phase. In adsorption phase, as the gas passes downward through the dryers, the water is absorbed by the molecular sieves. Required maximum water concentration in the dry gas outgoing dehydration facilities is 0.1 ppm mole. In order to dehydration is used 5A molecular sieve, and its problem is simultaneous adsorption of mercaptan and water. In the modeling process of adsorption, considering plug flow with axial dispersion model for gas flow pattern in bed, the mass balance equation have been used. In this system has been supposed that there are three components (mercaptan, water and inert component) for 5A molecular sieve and there are two components (water and inert component) for 3A molecular sieve. For each component is written a partial mass transfer equation and the overall mass balance equation for the system. Adsorption rate is approximated by linear driving force. Langmuir isotherm has been used to describe adsorption equilibrium on zeolit adsorbent. Simulation has been done in form isotherm. The set of partial differential equations of dynamic model is solved using the implicit finite difference algorithm. The concentration profiles and pressure profiles are presented in bed length. For corroboration of simulation, the results are compared with dehydration unit operational data and observed 2.36% error for 5A molecular sieve which shows close corresponding of model with operational data. Comparison of two types of molecular sieve (in operating conditions) indicates that molecular sieve 5A is although somewhat better than 3A molecular sieve, but due to the simultaneous adsorption of mercaptan is not suitable for dehydration. So this is better that 3A molecular sieve is used in process. In order to compare the effects of important operational parameters on system performance were considered 9 case studies for each molecular sieve. The results show that the effect of temperature and pressure is negliable on the rate of water adsorption, but increasing of bed height with larger molecular sieve decrease adsorption. Keywords : Adsorption, Dehydration, molecular sieve.