The dimensionless extinction constants, K-e, of soot produced from a small laminar flame burning ultra low sulfur diesel (ULSD) and soy methyl ester (B100) biodiesel fuel were measured in the visible (632.8 nm) and near infrared (1,064 nm) wavelengths. Experiments were performed at atmospheric pressure using a transmission cell reciprocal nephelometer (TCRN) in which simultaneous gravimetric sampling and light extinction techniques (GSLE) were employed. For the diesel soot, the average value of the K-e at 632.8 nm was 11.1 whereas that of the K-e for biodiesel was 11.8 at the same wavelength. As the wavelength increased up to 1,064 nm, the average K-e for diesel and biodiesel soot was found to reduce to 10.5 and 9.4, respectively. In an effort to quantitatively explain the variations in K-e (influenced by fuel type and wavelength), analysis of the influence of scattering, beam shielding, and nanostructure was performed through the measurements of soot physical and fractal properties and soot nanostructure properties. It was found that diesel soot was more closely aligned to graphitic properties, compared to biodiesel soot influencing the absorption component of dimensionless light extinction constant. Results also revealed that the influence of scattering was not a negligible component of extinction at 632.8 nm. However the influence of scattering decreases with wavelength from 632.8 to 1,064 nm, lowering the measured K-e values. The beam shield effects were observed to be an important mechanism that reduces the K-e for diesel soot at 632.8 nm and become weaker as the wavelength increases to 1,064 nm.