This study present experimental investigation of biomass gasification in the downdraft gasifier. Research was done in the real plant at various operation conditions. Since wood chips was used as fuel in the plant, the effect of fuel moisture on the gasification process performance was examined due high impact on the results. Air supply in the gasifier was divided into primary and secondary air flows to achieve higher syngas calorific value. The share of secondary air flow varied from 0 % to 15 % in the tests. Gasification plant was operated at different thermal capacity to find fuel supply influence on the gasification process performance. Experimental data analysis was done and gasification plant optimal operation setup was determined. Also regression models were developed to present syngas calorific value, gasifier capacity, cold and hot gas efficiency as function of gasifier operation setup.