In order to examine the effect of aromatic addition to Fischer-Tropsch Diesel (FTD) fuel on formation and oxidation processes of soot particles in diesel spray flame, small amount of naphthalene (0 to 65,000 ppm) was added to the FTD fuel and variation of soot morphology and nanostructure of primary soot particles directly sampled in a diesel spray flame were investigated via High-Resolution Transmission Electron Microscopy (HRTEM). A single-shot diesel spray flame was achieved in a constant volume combustion chamber under a diesel-like condition (Ta=1000K, Pa=2.7MPa) and a grid for HRTEM observation was directly exposed to the spray flame to thermophoretically sample soot particles onto the grid surface. The primary particle diameter, aggregate gyration radius, lattice fringe length, lattice fringe tortuosity and lattice fringe separation of soot particles sampled at different locations (from 60 to 90mm from nozzle tip) in the spray flame were analyzed. Naphthalene addition promoted soot formation process in the spray flame. The soot morphology analysis showed that the size of primary particles sampled in spray flames with and without naphthalene addition did not show significant difference, while the naphthalene addition increased size and the number of soot aggregates. These data suggest that naphthalene addition resulted in the increased total number of primary particles in the spray flame. The soot nanostructure analysis showed that the fringe separation became narrower from upstream to downstream for both cases with and without naphthalene, suggesting that the oxidation of soot particles progresses from up to downstream and thus the fringe layers get more crystallized. The change of the fringe separation from up to downstream was larger and faster in the naphthalene added case, suggesting that the naphthalene addition promoted faster formation of fluffier soot particles in the upstream and their faster oxidation.