An experimental study on an updraft gasifier (60 kW(th)), using large size biomass as fuel, has been carried out to obtain the temperature profile, equivalence ratio, gas composition and its heating value. A theoretical mass and energy balance has allowed a proper justification of the experimental results obtained. The gasifier behavior was studied in-depth using large size woody biomass to avoid the energy consumption required for chipping. The feedstock wood size was 1 m in length and 0.06 m in diameter and it was stacked 60 cm high, with a moisture content of 20%. Results were obtained for various flow conditions with air flow rates ranging from, 20 to 30 m(3)/h. Each test was carried out under continuous operation and steady state for 5 h. The different sub -processes taking place from top to bottom (grate) were under very transient conditions. Results for the temperature at the height of oxidation, reduction, pyrolysis and drying zones (23, 66, 78 and 92 cm, respectively) have been provided. The optimum gasifier behavior was reached with an airflow rate of 20.66 m(3)/h (or air inlet velocity of 1 m/s), leading to a producer gas heating value of 4500 kJ/N m(3) and a temperature of 955 degrees C at the oxidation stage. Comparison between the theoretical energy balance and thus derived from the tests have allowed for a better identification of the phenomena taking place during the particles conversion. Anyway, differences lower than 22% have been obtained in all the cases. (C) 2016 Elsevier Ltd. All rights reserved.