Butanol is an oxygenated renewable fuel and therefore is a potential candidate to be blended with gasoline to reduce particulate emissions. In this experimental investigation, particle number-size (PN-size) distribution and morphology (physical characterization) of soot emitted by the butanol–gasoline blend in a gasoline direct injection (GDI) engine have been investigated. The effect of engine load and fuel injection pressure (FIP) on particulates was investigated for two test fuels: gasoline and Bu15 (85%, v/v, gasoline blended with 15%, v/v, butanol) in a 0.5 L single-cylinder GDI engine using an engine exhaust particulate sizer (EEPS) and a partial flow dilution tunnel for collecting particulate samples on a filter paper. The physical characterization of particulates included primary particle size (Dp) and particle agglomerate characterization parameters such as agglomerate length (L), agglomerate width (W), skeletal length (Lsk) and skeletal width (Wsk), which were determined using a transmission electron microscope (TEM) and corresponding image analyses. PN-size distribution was relatively lower for Bu15, which decreased with increasing FIP. Regardless of the GDI engine operating condition, classical sphere and chain-like agglomerates having nearly similar nano-scale morphology were detected. The primary particle diameter changed with varying engine operating conditions. A comparative analysis of soot originating from Bu15 and gasoline was presented, which may be useful for gasoline particulate filter (GPF) design and to understand the regeneration of GPFs in practical engine applications.