Ultrafine particles of small mean diameter released from domestic coal combustion is an important parameter to consider as it affects air quality and human health. It is suggested that poor combustion conditions release particles of different sizes enriched with health-damaging chemicals such as polycyclic aromatic hydrocarbons. Furthermore, both smouldering and high efficient combustion conditions release particles, which are often carcinogenic. Information on fixed-bed domestic coal combustion char or soot particle size distribution (PSD) is limited, with many studies reporting on wood combustion. This study investigated the influence of coal combustion phases (ignition, flaming and coking) on particle number concentration and size distribution of ultrafine particles. D-grade bituminous coal was crushed to particle diameter of Ø 40–60 mm, combusted in a laboratory designed coal brazier (Imbaula) during experimental investigations of particle size distribution normalised to particle number concentration against particle diameter. Experiments were carried out using the reduced smoke top-lit updraft method, colloquially known as the “Basa njengo Magogo” (BnM) method. The tests were carried out in a laboratory-controlled environment. Particulate matter was monitored using a NanoScan Scanning Mobility Particle Sizer (SMPS). Particles from the top-lit updraft (TLUD) showed an ultrafine geometric mean diameter centred at approximately 109 ± 18.4 nm for the ignition phase, 54.9 ± 5.9 nm for the pyrolysis/ flaming phase, and 31.1 ± 5.1 nm for the coking phase. The particle mode diameter rapidly increased during the ignition phase (145 nm) and gradually decreased during the flaming phase (35 nm) and the coking phase (31 nm). This study shows that during smouldering combustion conditions (ignition): particle diameter increases, while as temperature increase the particle size decreases. The information is essential in estimating particle lung deposition and associated health risks.