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The structural change in co-flow propylene diffusion flames by irradiation of laser light is experimentally investigated to examine the effect of soot radiation at a certain position. The soot volume fraction and the flame/soot temperatures are measured by laser extinction and two-color-ratio pyrometry techniques. Transitions from a non-sooting to a sooting flame and vice versa are observed to depend on the irradiated positions of laser light. The structural change could be attributed to the following processes. When laser light is irradiated at the soot-formation region (lower part in the flame), the temperature of soot particles is increased by absorption of laser energy; the raised temperature enhances soot formation and increases the local volume fraction of soot. The increased amount of soot enhances the radiation loss and eventually lowers the flame temperature at the downstream direction. As a result, the soot-oxidation process is suppressed, and finally a non-sooting flame transforms to a sooting flame. The laser light irradiated at the competing section of soot formation and oxidation or at the oxidation section (upper part of the flame) also increases the temperature of soot particles, however, the increased soot temperature mostly enhances soot oxidation in these regions. Therefore, a decreased amount of soot lowers the radiation loss from the flame and maintains the flame temperature relatively high, compared with that without irradiation of laser light. As a result, the soot-oxidation process is more enhanced, and finally the transition from a sooting flame to a non-sooting flame occurs.