Butanol is a promising alcohol fuel. Previous studies on combustion and diesel engines showed different trends in sooting tendencies of the butanol isomers (n-butanol, iso-butanol, sec-butanol and tert-butanol).The impact of butanol isomers on the particulate emissions of GDI (Gasoline Direct Injection) engines, however, has not been reported. This work examines the combustion performance and particle number emissions of a GDI engine fueled with gasoline/butanol blends in steady state modes. Each isomer was tested at blend ratios from 10% to 50% by volume. Spark timings for all the fuels are set to obtain the maximum break torque (MBT), i.e. the MBT spark timings. Results show that the particle number concentration is reduced significantly with increasing butanol content for all the isomers. Sec-Butanol/gasoline yields the lowest particle number concentration, closely followed by n-butanol/gasoline, and then iso-butanol/gasoline, tert-butanol/gasoline yielded the highest particle number concentration. The particle mean diameter decreases slightly with the addition of butanol. Similar to the trend of particle number concentration, sec-butanol/gasoline has the smallest particle sizes while tert-butanol/gasoline the largest.The difference in the particle mean diameter of the four isomers is not significant. The rapid burning angle of gasoline and butanol/gasoline blends follows the ranking of laminar flame speed, while flame development angle is affected by both the laminar flame speed and flame stretch sensitivity. The flame development angle, the rapid burning angle and the cylinder temperature profiles of n-butanol/gasoline, sec-butanol gasoline and tert-butanol/gasoline are similar at the same blend ratio.