Biomass feedstocks such as agriculture residues, energy crops, woody biomass, and municipal solid wastes are used for gasification. The syngas composition of biomass gasification varies depending on a number of factors such as biomass type, temperature, particle size, gasifier type, and an oxidizing agent. The particle size and densification of the feedstock is especially critical to a fluidized bed gasifier for their proper fluidization with the gasification bed materials. The particle size not only impacts the formation of syngas product during gasification but it also dictates the selection of the gasifier. Tar is an undesirable and unavoidable product in the gasification process. Densification is a preferred way to improve gasification process especially regarding feeding biomass in the reactor because of the uniform shape, controlled particle size, and very low moisture. The effects of the physical properties of biomass on syngas compositions were discussed as well as the importance of physical pretreatments of biomass feedstock.