Abstract: Tar is a kind of very harmful product during the biomass gasification process, and it is not considered in the conventional thermodynamic mathematical model because of its components complexity. In order to research the effect of operation condition on ingredients and tar content of the fuel gas, a thermodynamic mathematical model of biomass gasification process with tar was developed based on material balance, energy balance and chemical equilibrium, taking the influences of system heat loss and carbon partial conversion into account. It is well known that the number of identified tar components is more than 100, and the components of quality ratio more than 5% are benzene, toluene, phenol, dimethylbenzene, phenylethylene, naphthalene, and so on. According to the quality equal principle of the main tar components, tar was indicated as the imaginary material C6H6.2O0.2 in the thermodynamic mathematical model. The chemical reactions selected in the model were from the deoxidization process as the reaction rate was much slower than the oxidization reaction. The model was solved with Newton-Raphson method, which was validated in comparison with literature data. In addition, the developed model was used to study the operating conditions, such as air preheating temperature, air equivalence ratio (ER) and steam ratio, on gas components and tar content. The results indicated that, with the increase in air preheating temperature, the tar content was decreased but the low heat value (LHV) was increased. When ER was increased from 0.2 to 0.3, the tar content and the LHV were both decreased. As steam ratio was increased from 0 to 10%, the tar content was decreased and gasification efficiency was improved. The results might provide valuable references for low-tar and clean utilizations of biomass gasification.