The fractal particle concept was applied to the real-time light transmission spectroscopy (RTLTS), an in-situ particle-size distribution measurement technique. The number of fractal-shaped particles in diesel engine exhaust gases was measured under transient conditions using RTLTS. The engine operated at a fixed speed and under different loads. One-second resolution RTLTS detected instantaneous changes in the particle emission under rapidly decreasing load conditions. The particle size determined with RTLTS was compared with values obtained using commercial analysis systems, such as a differential mobility analyzer (DMA) and condensation particle counter (CPC). The fractal reflected Mie-scattering database was applied to RTLTS to reduce multi-scattering error due to the fractal shape of the aggregates. The single-mode particle size distribution was obtained using reflective fractal analysis. Using a particle shape reflected light-scattering database can enhance the reliability of RTLTS measurements. (C) 2015 Elsevier Ltd. All rights reserved.