Butanol is a promising alcohol fuel. Previous studies in flames and diesel engines showed different trends in sooting tendencies of the butanol isomers (n-butanol, iso-butanol, sec-butanol and tert-butanol). However, the impact of butanol isomers on the particulate emissions of GDI (Gasoline Direct Injection) engines has not been reported. This work examined the combustion performance and particle number emissions of a GDI engine fueled with gasoline/butanol blends at steady state modes. Each isomer was tested at the blend ratio 10% to 50% by volume. Spark timings for all the fuels were set to obtain the maximum break torque, i.e. the MBT spark timings. Results showed that the particle number concentration could be reduced significantly with the increasing butanol content for all the isomers. Sec-butanol/gasoline yielded 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 decreased slightly with the addition of butanol. Similar to the trend of particle number concentration, sec-butanol/gasoline had the smallest particle sizes while tert-butanol/gasoline the largest. However, the difference in the particle mean diameter of the four isomers was not quite significant. The flame development angle and rapid burning angle followed the trend of the laminar flame speed of the fuels, and tert-butanol/gasoline blends showed the longest ones. However, the flame development angle, the rapid burning angle and the cylinder temperature profiles of the other three sorts of butanol/gasoline blends were close to each other at the same blend ratio. Thus, the differences in the particulate emissions of the blends of butanol isomers and gasoline could be mainly attributed to the diverse reaction pathways and intermediates rather than the fuel-air mixing.