Gasification of coal is a well-known technology used to convert solid coal into gas (syngas). The implementation of gasification for waste and biomass still requires attention due to the difference in nature of biomass compared to coal. Although, modification to a gasifier is one of the main approaches to achieve high quality syngas production (high H-2/CO ratio) and to eliminate tar formation, the effect of design of the gasifier has not been studied. Downdraft gasifiers are reported to produce relatively high quality syngas with low tar concentration compared to other designs. Therefore, in this study a 20 cm diameter throat downdraft gasifier was numerically optimised using Computational Fluid Dynamics modelling. The effect of throat diameter and the position of the air inlet nozzles above the throat on the properties of the gas and the temperature profile in the gasifier was systematically investigated and validated using experimental data. The throat diameter and the position of the air inlet nozzles had a significant effect on the properties of the gas and temperature profile. The modelling and experimental results agreed very well with less than 5% deviation. This confirms that the numerical approaches are valid and can be used in scaling up biomass gasification, reducing process development time from laboratory scales to pilot/industrial scales. The maximum concentration of H-2 (31.2%mol) and highest H-2/CO ratio (1.25) was found at a ratio of throat diameter to gasifier diameter of 0.40 and the position of the air inlet nozzles at 10 cm above the throat.