Systematic investigations of the effects of fuel compositions on sooting tendencies were performed in co-flow methane/air diffusion flames doped with different surrogate blends. Test surrogate fuels were blends of four components in the diesel surrogate palette. The laser-induced incandescence (LII) technique was applied to obtain the 2D soot volume fraction distributions. An apparatus-independent yield sooting index (YSI) was acquired experimentally. Analysis of YSI data and the soot volume fraction distributions reveals that the branching degree of alkanes did not have an obvious effect on YSI values while the addition of the aromatics provided a tremendous promoting effect on sooting tendencies of test flames. Besides, it could be speculated that increasing aromatic content dominantly contributes to the soot inception step and has a negligible effect on the HACA (H-abstraction-C2H2-addition) pathway or polycyclic aromatic hydrocarbon (PAH) condensation. Simulation works using a semidetailed chemical kinetic model were performed to get insight into the compositional effects on PAH formation. Simulation results showed that as the mole fraction of 1-methylnaphthalene increased, more direct pathways to form small PAHs were obviously strengthened and became dominant step by step, promoting subsequent growth of PAHs and soot inception and eventually enhancing the sooting tendencies. Furthermore, this work can provide guidance for diesel surrogate formulation of a particular sooting tendency using YSI as one of the fuel characteristics.