Ultrafine particles within Diesel exhaust gases have gained increased attention within the last few years, as especially the particle size is expected to be the key property for possible toxicity and carcinogenicity from Diesel engine emission. Various exhaust aftertreatment systems like oxidizing catalysts or soot filters have been applied for a significant reduction of soot mass concentration, but their influence on particle sizes and number concentration also has to be considered. This enhances the need for new measurement techniques which allow to measure relevant morphological parameters of soot particles.Laser-induced incandescence (LII) is presented as a favorable optical technique for soot measurements. It is based on the analysis of the enhanced thermal radiation of the particles after heating with an intense laser pulse and has been successfully applied for soot concentration measurements within laboratory flames and even within technical systems like the cylinder of a Diesel engine. Also, a great potential for emission measurements has been indicated.In this work, the soot particle emission of a turbocharged 1.9l DI Diesel engine has been investigated within the raw exhaust gas. Quantitative on-line measurements of both the primary particle sizes, based on the analysis of particle cooling, and the mass concentration are simultaneously feasible, even under very low sooting conditions. Basic features of the technique are discussed, and results are presented for various engine speeds and loads and additionally for cold start conditions. Primary particle sizes in a range of 19-36 nm and mass concentrations of 5-110 mg/m3 were found, and a characteristic dependence on the operating conditions of the engine was observed.