<p>An experimental study was conducted on the effect of the shape of the fuel nozzle outlet on the suppression of soot emission from a coaxial flow diffusion flame. Three nozzle shapes were used, namely circular, square, and an equilateral triangle. Propane was used as the fuel and air as the oxidizer. The fuel flow rates Q_f were 2, 3, and 4 cm³/s. At the maximum Q_f of 4 cm³/s, the nozzle shape changed the soot discharge characteristics and the flame structure. The soot discharge was suppressed the most with the triangular nozzle and the least with the circular nozzle. The shape of the soot concentration distribution near the flame tip was similar to the nozzle shape, indicating that the influence of the nozzle shape reached the flame tip. Soot formation started earliest in the triangular nozzle and latest in the circular nozzle. In non-circular nozzles, soot started to form from the downstream regions of the corners of the nozzle rims. This is because the profile of the fuel flow was close to the flame front in these downstream regions. Soot oxidation was fastest in the triangular nozzle and slowest in the circular nozzle. Therefore, the flame was the lowest in the triangular nozzle and the highest in the circular nozzle. The oxidizer flow velocity toward the flame base was high in the downstream region of the flat side of the non-circular nozzles. The velocity was especially high in the triangular nozzle, and the CH radical emission, which is closely related to the heat release rate, was also strong at the flame base. As mentioned above, the fuel concentration distribution, the convective transport of the oxidizer, and the flame structures changed in the non-circular nozzles, and in particular, soot emission was suppressed in the triangular nozzle.</p>