A combination of transmission electron microscopic imaging and semi-automatic image analysis allows detecting and measuring the primary particles of aggregated nanomaterials. The median primary particle size, the number based size distribution, the percentage of nanoparticles and the volume specific surface area of powdered nanomaterials are determined in the framework of the European Commission's nanomaterial definition. The principle of a semi-automated approach to measure the minimal size in one dimension of primary particles in aggregates by transmission electron microscopy is developed using the model of the powdered, aggregated titanium dioxide representative test nanomaterial NM-100 with a mean primary particle diameter near the 100 nm limit. The primary particles are detected based on watershed segmentation and their minimal size and overlap coefficient are measured based on an Euclidean distance map. The primary partides of NM-100 contain 22% nanoparticles, with a size between 1 nm and 100 nm. The method is evaluated to measure other aggregated TiO2 synthetic amorphous silica and carbon black nanomaterials for which the percentage of nanoparticles is nearly 100%. A high level of automation allows measuring the diameter of the maximal inscribed circle efficiently. The maximal inscribed circle is a measure for the minimal primary particle size in one dimension and is shown to be commutable with Feret min measurements. The standard method to determine the fractal properties and the volume specific surface area of fractal-like aggregates is refined supporting on the diameter of the maximal inscribed circle and the overlap coefficient measurements for each individual primary particle. (C) 2014 Elsevier B.V. All rights reserved.