Toxic gases are significant in fire risk evaluation. Previous studies have focused on their characteristics by piloted ignition. Spontaneous ignition is a complex phenomenon that combustible materials are ignited by internal heating, without the spark plug. Comparing with piloted ignition, process of spontaneous ignition is much closer to the development of real fire. Therefore, carbon monoxide (CO) of six species of wood samples under external heat flux by spontaneous ignition in a cone calorimeter was investigated. Results showed that influence of thickness to peak CO release rate can be ignored, but time to peak is postponed with a higher thickness. Peak CO release rate decreases with a higher external heat flux, but the decrease is not obvious when heat flux increases from 50 to 75 kW/m(2). The flame also has influences to the CO release rate. A sharp decrease of CO release rate happens shortly after ignition and a second peak is near the end of the experiment. Moisture reduces CO release rate and postpones time to peak CO release rate. An empirical model of CO yield of wood samples under different external heat flux and moisture content by spontaneous ignition was developed. This empirical model can be used not only for fire risk evaluation, but also for modeling input and validation. (C) 2012 Published by Elsevier Ltd.