The spray characteristics of bio-oil for combustion were investigated by phase Doppler anemometry (PDA) with Sauter mean diameter (SMD) as evaluating indicator. The effects of air liquid mass flow ratio (ALR) on SMD were analyzed. The SMD distributions along axial direction of bio-oil, diesel and bio-oil/ethanol blend spray droplets were also compared. The results showed that SMD of bio-oil spray droplets exhibited a tendency to first decrease and then increase along axial direction and increased continuously along radial direction. The SMD diminished with the increase of gas-liquid flow ratio and had little variation when the gas-liquid flow ratio exceeded a certain value. Among three kinds of fuels, the atomizing quality of diesel was the best, while that of the bio-oil was the worst. By adding ethanol to the bio-oil, the atomizing quality was improved. The empirical equation for the SMD was derived. The predicted results were found to be in a good agreement with the experiment results, which confirmed the reliability of the empirical equation and would offer information for designing bio-oil spray nozzle and optimizing bio-oil combustion conditions.