Biomass is being used widely for energy production in various ways. One of the main schemes for energy production from biomass is gasification, and a powerful tool for optimising the behaviour of biomass gasifiers and also obtaining the required design for specific working conditions is the modelling. An integrated kinetic model was built by the authors ([1], and [2]), which enabled to simulate, design and optimise the processes of downdraft biomass gasifiers for the production of high quality syngas. In the current research presented in this paper, the kinetic code has been extended by including detailed chemical mechanisms for the tar evolution and formation inside the gasifier. The model incorporated the evolution of 4 main tar species from pyrolysis to combustion and gasification with the formation and kinetic reaction rates of benzene, naphthalene, toluene, and phenol tar compounds. The effect of varying moisture content (MC) and equivalence ratio (ER) is studied to find out the optimum working conditions leading to the reduction of tar amount in the producer gas from downdraft gasifiers. The results reveals that moderate ER of 0.3, with lower MC<10%, leads to the production of higher quality syngas with lower tar amounts.