Abstract: Calculating water vapor contents of gas phase accurately is a base of complex phase behavior analysis in high temperature and high pressure (HTHP) gas condensate reservoirs. In high temperature and high pressure gas condensate reservoirs, water and hydrocarbon molecules in gas phase are associated to form hydrocarbon carbohydrates (ACnHm·BH₂O). This is the major reason for special characteristics in gas phase. Based on principles of phase balance and fluid thermodynamics equilibrium, taking into account the influence of high temperature, high pressure and the density on the associative reaction, association model with four parameters for calculating water vapor contents was established. The values of four model parameters (a_i) were determined using multi-variable regression method. The new method is demonstrated reliably and practically by analyzing and comparing some examples.